DISCLAIMER: Iron Man 3, The Incredible Hulk, Captain America: The First Avenger, and The Avengers are the properties of Walt Disney Studios, Paramount Studios, and Marvel Studios. This work was created purely for enjoyment. No money was made, and no infringement was intended.

RATING: T (for language, violence, disturbing imagery)

AUTHOR'S NOTE: So we did Steve and Tony ("Invincible") and Steve and Clint ("The Right Call") and Steve and Clint and Tony ("The Last Level") and Steve and Thor ("Self from Self") and Steve and Natasha ("Red Rain"). This is Steve and Bruce (with Tony and Clint in strong support). It's nestled after Iron Man 3 and Thor: The Dark World but before Captain America: The Winter Soldier (spoilers for the first two but no spoilers for the latter). Of course, tons of whump and angst to go around. No pairings aside from Tony/Pepper (and Science Bros. bromance :-D). Parts of this story are canon with MCU, parts are canon with the comics, and parts I'm making up as I go (which might make this a tad AU? We'll see).

Also, my science is based on a mixture of how things might work, how things could work in comic-verse, and how the story needs them to work. So take it all with a grain of salt! Onward and enjoy!

UNSUSTAINABLE

1

Bruce Banner liked to stay out of the spotlight.

Therefore his friendship with Tony Stark was a little counterintuitive.

Tony was flashy and wealthy and extravagant and pretty much the polar opposite of Bruce in every way. Where Bruce was quiet and calm and reserved, Tony was loud and easily riled and opinionated. Where Bruce appreciated simplicity and anonymity, Tony craved complexity and notoriety. Tony actively sought out trouble, and Bruce did his damnedest to avoid it. Tony flourished with attention, and Bruce shriveled and floundered when people noticed him. They went together about as well as oil and water (with Tony as the highly combustible part of that mixture prone to explosion and disaster). If it wasn't for their mutual love of science, they'd really have nothing in common. Still, they'd inexplicably become friends.

Bruce still doubted his sanity sometimes. He really did.

"I'm kinda worried this thing is gonna blow up in my face." Tony was fiddling with the boot of his newest Iron Man suit (Mark 50 or something of those lines – honestly Bruce had lost count). And honestly Bruce had stopped listening a few minutes ago. Tony was prone to rambling and babbling nonsensically while he worked. That was another huge disparity in their personalities. Bruce actually appreciated peace and quiet, and there wasn't a whole lot of that around Stark Tower. He wondered sometimes if it would kill Tony to sit still for a moment and not talk. He wondered if he was even capable.

Stark sat on a stool, chewing on a piece of pepperoni pizza, staring at the innards of the boot. He had grease on his shirt (pizza or otherwise – Bruce didn't know). His face was tight with concentration, but his eyes suggested he really wasn't focused. At least not on one thing. Tony was brilliant, maybe the smartest man Bruce had ever known, and he was a phenomenal multitasker. He could simultaneously do a dozen things, his agile mind running much faster and more efficiently than his body (hell, than most computers), and not drop the ball on any of them. Well, a dozen things related to inventing and designing and building stuff. He was pretty bad at most other things: running his company, keeping his act together, keeping his life in order. Keeping Pepper happy (though that one was not for lack of trying, and since the Mandarin incident, things had definitely improved on that front).

But, then, Bruce was hardly one to judge. He wasn't exactly stellar at keeping his life together. Although, again, things had improved on that front since the Battle of New York a year and a half ago. A large part of his current stable state was due in no small part to Tony. The man had opened his home and his labs and his life to Bruce without ever making a show about it (which for Tony was saying something). They'd just fallen into each other, an easy relationship (the first Bruce had had in what felt like forever). Tony took him at face value, and the issue of the "Other Guy" was never an issue between them. Most of all, Tony wasn't afraid of him. That sort of implicit trust was a soothing balm for someone who never got close out of fear that the monster within would escape his control and hurt anyone dumb enough to be by his side. They spent a lot of time together, inventing together, working together, tinkering together. They'd had their moments apart (sometimes a lot of moments, weeks or months at a stretch), but when Tony was done with his latest life crisis or Bruce had completed his last guilt-ridden or fear-driven trip into self-exile in some remote part of the world, they always found each other again. There was a standing invitation at Stark Tower, and though Bruce had been reticent at first about staying some place so visible in the middle of some place so populated, it didn't bother him so much anymore. Tony grounded him, and he needed it.

And Tony needed him, too. It was heartening, damn nice when he admitted it to himself. Extremis would have killed Pepper had it not been for Bruce's help in neutralizing it. And Extremis had finally "burned away" some of Tony's demons and allowed him to at long last remove the shrapnel from his heart and the arc reactor from his chest. Bruce had been instrumental in all of that, and he was secretly very proud that he had been.

"You're drifting again," Tony sing-songed from the other side of the desk. "You're a crappy listener. You know that, right."

Bruce rolled his eyes and went back to looking over his latest project on one of Stark's many holographic computer terminals. His last trip to India had inspired him to investigate the possibility self-sustaining plants given the region's level of famine and poverty. He'd seen too many people starving, too many children with no meat on their bones and swollen stomachs and dead eyes, to continue to turn a blind eye to it. Nothing any nation could do seemed to be enough to feed hundreds of thousands of poor families in the world, so Bruce had taken it upon himself to do what he could. Everything he had tried to this point, practicing medicine for those who couldn't afford it, offering to charity to everyone he could, had been like slapping a band-aid on a mortal wound. So he'd turned to science because that was what he did. And tying science into something he wanted to do made him only want to do it all the more, so he'd spent the last couple of weeks at Stark Tower, working on his newest endeavor.

But, unfortunately, he was reaching an impasse. His thought had been simple. Extremis conferred extraordinary resilience, unbelievable strength, the capability to veritably regrow damaged tissue. Its capabilities were not well studied, even, so there was theoretically no limit to what it could do. However, the side-effects were something of an (actually a huge) issue. Madness. Rage. Not to mention the heat and energy produced by the Extremis reaction with organic tissue that basically resulted in combustion of said tissue. Bruce had developed a way to neutralize that aspect of the chemical, but with that went a lot of its positive qualities. The preternatural strength. The regenerative abilities. Some of Extremis functioned even with the dampening qualities of the neutralizing agents, but not enough to create plants that bloomed and bore fruit indefinitely and were immune to age and disease. And when he lessened the amount of neutralizing agents, his plants went up in a small, sad ball of flame and smoke.

He had been trying to look over the latest round of data. His last tomato plant had survived a few days, sprouting some large and plump and promising fruit, before going the way of all the other plants. He was getting a little discouraged, frankly, and more than a little frustrated. The scientist in him knew that Extremis had a purpose, that maybe the rogue group Advanced Idea Mechanics (or AIM) had used it for evil but it could be fixed and redeemed. He probably should have given up, but he couldn't let it go. He and Tony were too alike in that regard. When they knew they were right, they'd do just about anything to prove it. Pepper called it being incorrigible. Tony called it being confident. Bruce just thought it was being a good scientist.

"Earth to Brucey," Tony called. "Houston, do we have a problem?"

"What? Oh. What were you saying?"

"I was saying that I think any marriage proposal I make to her is gonna blow up in my face. I said it about three times in fact." Tony pushed himself away from his work bench, his stool rolling across the floor as he reached for another piece of pizza from the box atop the adjacent bench. Food didn't really belong in a workroom, but Tony wasn't exactly the type to care about the rules. "She's expecting it now, and she likes things to be unexpected. Now how the hell am I supposed to deal with that kind of logic?"

"Oh. I dunno."

Tony rolled his eyes. "You're useless. How many times a day do you tune me out?"

"Is that a rhetorical question?"

"You need to get better at this whole listening thing."

"What I need is a stabilizing agent," Bruce answered, shaking his head and rubbing his chin at the latest sets of simulations on the screen before him. "None of this stuff I've tried has come even remotely close to working. It's all too weak."

Tony took a messy bite of his pizza, grumbling the whole time. "I could buy her anything, take her anywhere, but somehow she'll know what it's about right away. She's smart like that. So that leaves just doing it in the middle of her brushing her teeth or something, but then she'll complain that it wasn't special enough. I'm screwed either way." Bruce narrowed his eyes and watched as the computer projected out how long his tomato plant might survive if he varied his lasted concoction of stabilizing chemicals. It was downright pathetic how insufficient everything he had was. "I don't even know why I'm asking you. The longest relationship you've had in months was with that plant, and you killed it."

Bruce sighed and dropped his head onto his folded forearms. "Back to the drawing table. Scratch another one off the list."

"How long is the list?"

"Short and getting shorter."

"It is possible, contrary to your hopes and dreams, that Extremis is just fundamentally a piece of shit. If they had been able to make it work, I wouldn't have had to fix it." At Bruce's withering look, Tony conceded. "We wouldn't have had to fix it. Not that Killian and his band of morons were all that smart. Hell, I fixed part of the problem when I was fall-down drunk off my ass." Tony stuck a probe into the boot in front of him after stuffing the remains of his pizza into his mouth. "Letting it go wouldn't be so bad. You're starting to smell like scorched fertilizer."

"Your advice is, as always, much appreciated," Bruce dryly remarked.

"What I'm here for," Tony said with a patented shit-eating grin. "If your advice to me was half as awesome as my advice to you, I'd say this is an equal partnership."

Bruce sighed. Maybe Tony was right. He'd been turning this problem over and over again in his head and losing sleep over it and agonizing over it for weeks. He didn't like things he couldn't solve or even understand. He didn't like giving up, even if everybody thought he was mellow and malleable and unobtrusive. He was used to things coming easy to him, being obvious and readily apparent. Maybe it wasn't worth the frustration. It still surprised him sometimes that he was such a poor judge of what was deserving of his anger and what wasn't. But his baseline of "angry" had shifted so much in the last few years. "Look, I highly doubt she really cares how it happens so long as it happens. You're over-thinking this."

"Now that is the very definition of the pot calling the kettle black," Tony remarked.

Bruce gave him another long-suffering glare. "Pepper knows you love her, right?"

"Well, yeah. I mean, I gave her a necklace made of shrapnel. It was a gift that literally came from my heart. If that doesn't say 'I love you', I don't know what does."

Bruce rolled his eyes. "If she knows you love her, it'll be fine. She'll be happy no matter how you ask her."

"Never pegged you for the romantic type," Tony said. He was deflecting, but Bruce could see his face relax just a little and a light of pride and relief come to his eyes. "Did the Hulk shrivel inside you even a little bit when you said that?"

If it had been anyone else, that would have bothered him. The mere mention of the "Other Guy" used to really set him on edge. But with Tony, it was just one of those things. Teasing and ribbing and testing were things that Tony just did. He fiddled with dangerous stuff just to see what would happen, not because he was cruel or malicious or self-destructive (well, not entirely because he was self-destructive). He pushed buttons to learn and stimulate and bring down walls and force things out in the open. It was the sort of no-holds-barred, fly-by-the-seat-of-your-pants approach to life that terrified him after his accident. Now it was just another thing on another day. "Just do us both a favor and propose already? You have been whining about this for weeks."

Tony had the decency to look somewhat affronted. "Whining? I don't–"

"Sir." JARVIS' calm voice interrupted their conversation, echoing slightly in the spacious workshop around them. "Captain Rogers and Agent Barton are here."

Tony's face immediately tightened in frustration and irritation, and he set his tools down to the work bench with a clank. "No. Oh, no. Tell them no. No way in hell. I am done. No more. And I already explained to Fury that my consulting hours were–"

"They are already in the elevator and headed to you," JARVIS declared.

"On whose authority?" The doors to the elevator on the other end of the lab dinged and slid open. Tony sighed and sagged on his stool in defeat. "Betrayed by the only person I thought I could trust."

Pepper Potts was laughing at something as she stepped out of the lift. She looked positively radiant, dressed in an expensive black pantsuit and a white blouse. Her auburn hair was pulled into a loose bun, wisps and tendrils of orange falling loose to frame her pretty face. Her red lips were pulled into a sweet smile as she hung onto the arm of Steve Rogers. The soldier's affable grin slid away when he saw Tony. "Don't tell me you can't find yourself a date, Steve," Pepper said as they walked out into the lab. Obviously they had been catching up on the ride up through the tower. "I happen to know dozens of women who would fall over themselves for a chance to go out with you."

"I doubt that, ma'am," Steve stammered.

Pepper laughed again, refusing to let go of Rogers' arm as they strolled across the large room to the work area. "Sure they would! Don't be so modest. You're Captain America, for crying out loud. That's one in a million. Strong. Handsome. Wholesome." Steve actually blushed and was looking increasingly uncomfortable. "I'd be after you in a heartbeat if it weren't for Tony."

"What the hell? Rogers, first you invite yourself over and now you flirt with my girl?" Tony accused, standing from his stool.

Rogers winced and shook his head. Pepper smiled again. She was a shrewd woman, smart and beautiful and capable, and she didn't put up with anyone's nonsense, least of all Stark's. She removed her arm from the crook of Steve's elbow and walked over to Tony, her eyes gaining a mischievous twinkle. She wiped the grease from his goatee with a napkin she grabbed from one of the benches (naturally miles away from Tony himself) and then slid her arms around his neck. "No, no, babe. I'm flirting with him."

Next to Steve, Clint Barton groaned.

Pepper's face gained a harder edge. "And 'my girl'? That sounds pretty official, doesn't it? Something that official–"

Tony shook his head and pulled her away. "No way. You're in charge of everything else in my life, Potts. You're not in charge of this."

"Just a friendly reminder."

"Consider me reminded. You're making this impossible, by the way."

Pepper smiled. "I have meetings. Play nice with your friends." She gave Tony a quick peck on the cheek and then walked away, stopping beside Steve and Clint. "Thanks, Captain. Agent Barton."

Steve had his hands clasped together in front of him. He managed a grin and a nod. "Ma'am." All four men watched her stride back to the elevator and step inside it. Then the doors closed and she was gone.

"I think that is what we call lighting a fire under your ass," Barton said.

Tony sighed and plopped back down on his stool tiredly. He wasn't embarrassed; he didn't have the capacity to be embarrassed. He wheeled himself back over to his work. "Good to see you guys," he said, but his tone suggested he wasn't at all pleased with their sudden appearance. Tony glanced out of the corner of his eye toward the two men. Clint was dressed in black with a SHIELD jacket and a holstered gun on his hip. Bruce wondered how many other weapons the master assassin had hidden. Steve sported a new uniform that was dark blue and trimmed in silver with a gleaming gray star over his chest. He carried his shield on his back. "Nice threads. Definitely an improvement over the spangly outfit. To what do we owe this patriotic pleasure?"

Steve gave a humorless smile. It had been a while (almost a year?) since Bruce had seen him last. He looked strikingly different. More confident. More modern. His hair was shorter, his stature maybe a bit taller, the set of his face and the light in his eyes surer of his place in this new world. He looked every bit like Captain America, a living war legend and the leader of the Avengers. It would be a lie to say they knew anything about each other; despite working together to repel the Chitauri invasion, they were hardly more than acquaintances. The Steve Rogers he'd met before the Battle of New York had been sad, serious, and visibly suffering from being lost and presumed dead fighting HYDRA during World War II only to be found seventy years in the future. This Steve Rogers was still serious, at least, but calm and collected and grounded. Bruce had to admire that. Hell, Bruce admired a lot about him, if he could be honest with himself.

But they had no common ground between them, nothing beyond a few frantic minutes they'd shared together to save the city. And Steve intimidated him. Steve was the world's one and only super soldier, transformed from a sick, weak kid into one of the fastest, strongest, and smartest men on the planet. The serum that had performed this huge scientific feat back during World War II had been long lost, and no attempt to recreate it had been successful. Most, Bruce's own included, had ended disastrously. And it wasn't just that Steve was a reminder of how hellishly wrong his work on the serum and Gamma radiation had gone. Steve was the symbol of why it hadn't worked. Steve was all parts valor and courage and kindness. He was the exemplar hero, strong in the face of injustice, noble and self-sacrificing, compassionate and true. He was everything that Bruce wasn't. One of the tenants of the super soldier serum was its ability to amplify everything within a man. With Steve, it had taken a frail boy's huge and powerful heart and turned him into a warrior for peace. A shield between the innocent and those who tried to harm them. With Bruce, it had taken his anger and arrogance and turned him into a monster. Steve represented a stark and undeniable truth: it hadn't been his science that had been so fatally flawed. It had been him.

That was somewhat difficult to come to terms with. Years later, he still hadn't.

Steve's face was very no-nonsense but not at all rattled, even by Tony's sarcasm. "We're here on behalf of SHIELD."

"Yeah. Got that part, Captain Obvious."

Bruce shook his head. Maybe Tony wasn't surprised, but he sure was. Especially since the last time he'd seen Steve talk about trusting SHIELD he had been condemning them over the HYDRA weapons he'd found aboard the helicarrier. "I wasn't aware you were working for them."

"Remember what I told you about needing to improve your listening skills?" Tony cocked an eyebrow. "And I thought you weren't marching to Fury's fife, Rogers."

Steve didn't react to the bait. He released a slow breath, his tall, muscular form deflating softly. "He asked me for my help a little while ago. I agreed so long as SHIELD stays true to what it's meant to do."

Tony grunted dismissively at that. "I'd count all the ways that's monumentally naïve, but we don't have all day. What national security goal brings you to me this time?"

"Believe it or not, we're not here to talk to you." Barton stepped up beside Rogers, tossing a USB thumb drive toward Stark. Tony caught it, a look of surprise shattering his previously nonchalant face. Clint turned and appraised Bruce evenly. He was even harder to read than Steve, a truly cool customer. Steve might have visibly changed, but Barton looked exactly the same as he had before. Confident. Guarded. As little as Bruce knew about Rogers, he knew even less about Hawkeye other than he'd spent the vast majority of the Chitauri mess under the control of a deranged demigod and thus playing for the bad guys. The man exuded deadliness with each and every stern look, including the one now stoically analyzing him. "We're here to talk to Doctor Banner."

The room was silent for a long minute. Bruce couldn't get his head around that at first. And when he did, something inside him, that little voice of warning that he'd learned to trust over the years, started chanting in his head that this was bad. "Me? What do you want with me?"

"Right now, just your help," Clint answered.

"Great. The last time Fury wanted my help most of midtown Manhattan was destroyed. You guys probably remember that."

"Nothing on that scale," Clint assured.

Tony jacked the USB drive into one of JARVIS' computer terminals, and a slew of data appeared. He reached into the holographic display and grabbed footage of a man giving a symposium at New York University on genetic engineering. Bruce recognized the face immediately, even before Tony brought up the SHIELD profile on him. "That's Dan Lahey. He and I were post-docs together at Culver…" A sinking sensation settled in the pit of his stomach. "What is it that you think he's done? Because whatever it is, he didn't do it."

"What makes you say that?" Steve asked, folding his impressive arms across his chest. It wasn't a manipulative question. He simply wanted Bruce's take.

But Bruce couldn't help but be defensive. "The guy's a mouse. Brilliant biochemist but pretty much a pushover. He had a chance to work on a grant for the CDC developing vaccines for some pretty serious stuff, anthrax and H1N1 and the like, but he refused because it would involve experimenting on animals. And not because he's a card-carrying PETA member or anything like that. He just didn't think he'd have it in himself to hurt a fly." Clint and Steve shared a quick look that Bruce couldn't read. He didn't appreciate the secrecy. He felt himself getting riled, so he drew a deep breath. "What's this about?"

"You're meeting with Lahey tomorrow, right?" Clint asked.

Bruce didn't like where this was going. "Yes? I don't see what that has to do with anything." When neither of the SHIELD agents said anything further, he got more irritated. "He invited me to his lab to consult on one of his projects. Tony was going to come with me."

"Yeah, I was going to go with him," Tony said.

"We know," Clint said. "We'd like for you to allow us to come as well."

"You know? How the hell could you know that?" Bruce asked. His patience was wearing thin, and he was feeling increasingly exposed. He never liked that feeling. "I suppose it's pretty damn naïve to think only the NSA is spying on my email. It's open season on American freedoms."

Clint shook his head. "SHIELD wasn't spying on you," he corrected. "A few weeks ago, Lahey received a rather large grant from NIH for some project supposedly based on studying the biochemical underpinnings of human emotion. That's what he called you to advise him on." Bruce nodded. Dan was a downright genius when it came to biochemical and genetic engineering. He'd excelled in his postdoctoral studies, quickly becoming a world-renowned expert on human cellular processes. But he'd never made much of himself after it became obvious that he had some rather… unorthodox ideas, namely that human emotions could influence biochemical reactions. That human emotions could fundamentally alter chemical processes. Emotion was a rather subjective construct, one that was not easily or definitively or even quantitatively measurable, so what he was proposing, while intriguing, was pretty much cast aside as nonsense. Or at least not feasible. It didn't help that the man had about as much social grace as a shy five-year old. At the time, Bruce had humored his friend's interest in the human psyche as a powerful influence of biology mostly because he'd become something of a social and scientific pariah and Bruce had felt bad for him. But even then he'd thought Dan's ideas were pretty far-fetched.

Since becoming the Hulk and having his own rage turn his body from human to monster, his opinions had changed somewhat.

"Look, ever since the mess with Aldrich Killian, SHIELD's been keeping closer tabs on the scientific community. Fury will never admit it, but the whole AIM incident caught him a bit by surprise. He's had people keeping an eye on the loner types in the world of biochemical, genetic, and weapons research, trying to catch the next nut job before he invents the next WMD or worse," Clint explained. "We've been monitoring Lahey's email. He showed up on the radar after getting this grant. It's big and it's not legitimate. Nobody at NIH remembers ever receiving a proposal from him for this type of research."

"Well, there are politics at NIH, you know," Bruce countered. "Science may be a pure endeavor for the betterment of humanity, but scientists are human, and humans need to eat and have roofs over their heads and tend to get swallowed up by their own agendas. Dan lost tenure at Culver a couple years back. Research takes money. After a while you don't worry so much where it comes from so long as it keeps coming."

Tony was still picking through the data. His brow furrowed in puzzlement. "I can see what tipped off Fury," he commented. A few waves of his hands brought up a visual representation of a chain of money and the people involved in transferring it that was quite a few layers deep, from Lahey through program directors at NIH through a senator or two to a trust that funded a think tank and finally to one Maya Hansen. "Damn, I guess she got around." Tony's voice was pinched in just a bit of hurt and betrayal. "AIM had its grubby fingers pulling a lot of strings."

Bruce looked at the graphical representation of a conspiracy. He didn't like it, but it sure seemed like Dan was possibly involved with the fringe scientific community. The sort that made serums to regenerate the human body from crippling or devastating injuries but really just resulted in rage, chaos, and destruction. The sort that kept trying to recreate the super soldier serum no matter how many times it failed and how many innocent lives were lost in the process. He wanted to deny it because this didn't feel right. And he didn't want to consider why Dan really wanted to see him if he was involved with AIM. "Just because he was getting money from AIM doesn't mean he was working for them. The sort of research Dan was interested in doesn't tend to get funded from respectable organizations."

"Too dangerous?" Steve asked.

"No," Bruce said. "Just too out there. I'm sure he didn't know where the money was coming from. For crying out loud, he works at the Hopkins Research Institute. They're–"

"Not dangerous," Tony supplied. "Not even sexy. That place is where ideas go to die."

"The most exciting scientific breakthrough coming out of there in the last five years involves fruit flies."

Steve sighed slowly. "Doctor Banner–"

"Bruce," Bruce corrected tiredly.

Steve winced. "Bruce. You're right. It's probably nothing. And you're right about Doctor Lahey; he hardly seems the type to be caught up in something like this."

Clint added, "The guy doesn't even have a parking ticket on his record."

"And Fury shouldn't have been spying on him without warrant or at least without cause and exigent circumstances. But he was, and now that this has come to our attention, we can't just turn a blind eye," Steve said. "Maybe he's legitimate but AIM is trying to target him, trying to turn him into another scientist serving their ambitions like Hansen was. Like you said, the promise of money and fame and notoriety, especially when you've been ridiculed for most of your career, is alluring."

"It wasn't to Dan. He couldn't have cared less what people thought. He cared about the science."

Steve glanced at Clint. "That may well be the case. If it is, let's keep it that way. Let us escort you to the meeting tomorrow."

Bruce felt a little off-put by this whole conversation, first that SHIELD thought his friend was capable of being influenced by evil (or doing evil himself) despite his assurances to the contrary, and second that SHIELD didn't trust him to handle it. "Dan knows who I am," he said, shaking his head. "And he already knows Tony is coming. You really think he's going to try anything with Iron Man and…" He still couldn't get used to saying it; it was a damn pathetic defense mechanism, like if he didn't quite admit to it, it would go away. "… the Hulk standing right there?"

"He'll be even less inclined with Captain America and Hawkeye there as well," Steve said. "And, no, I don't anticipate anything happening. I don't anticipate that having the Avengers there is at all necessary. Director Fury asked us to go with you to make SHIELD's presence known, both to Doctor Lahey and to anyone seeking to manipulate or coerce him. That's it."

"That's it?" Tony repeated doubtfully. "He just wants you to stand there and look threatening?"

Clint and Steve shared another look. Barton crossed his arms over his chest and shifted his weight slightly. "Pretty much."

"You have my word," Steve said.

Bruce grimaced before slumping slightly in defeat. "You're not going to take 'no' for an answer, are you."

But Steve surprised him. "Yes, I will. I don't march to Fury's fife." He darted an annoyed glance at Stark. "However, I'd appreciate it if you let us do what we were sent to do. If AIM or whatever is left of it is targeting your friend, he's in danger."

Bruce looked between Steve and Clint. Were it anyone else, he'd worry that that comment was meant to appeal to his worry, to stoke to life old feelings of friendship in order to manipulate him into agreeing. But he knew Steve Rogers wouldn't resort to lying or tricking him. He was too good for that. If he was promising something, he meant it with every ounce of loyalty and sincerity. Bruce looked over at Tony. The inventor cleared his workspace of the data on Dan from SHIELD and shrugged neutrally. "Your meeting."

Stark was positively useless sometimes. Bruce turned his eyes back to the two SHIELD agents before him. He had to admit that the evidence that something was not entirely right with Dan's work and funding was pretty compelling. If SHIELD's presence could ward off interested parties, maybe it would be worth it. It had been for him. SHIELD had kept the US military and who knew how many other groups (both good and bad) off his tail for years. Maybe he'd be doing Dan a favor.

"Okay," he said. He watched Steve and Clint share yet another look – what was this? Some sort of SHIELD silent communication? "Just keep your distance, okay? Dan's not… well, he spooks easily. And he asked me to help, so I don't want him to think…"

"Got it. Won't be a problem," Steve promised. He smiled genuinely for the first time during the conversation. "Tomorrow at four o'clock, right?"

"Yeah. You guys can't invite yourselves to dinner afterward, though," Tony said. "That's plain rude. Now skedaddle. We're doing science."

Steve nodded. If he was relieved or worried or happy or proud of himself or whatever, it wasn't obvious. Neither was Clint's reaction. The two of them were like statues. "Thanks, Doctor Banner. Mr. Stark." They turned and left.

When the elevator doors were safely closed, Tony groaned. "Some things never change," he grumbled. "That guy still acts like he has an American flag, pole and all, stuck up his ass."

"I thought the two of you buried the hatchet after we saved the city."

Tony grabbed a probe and stuck it back into Iron Man's boot. "We did. Doesn't mean that I have to like him, though. What did Pepper say?" He did the worst impression of Pepper's voice imaginable. "So strong and handsome and wholesome." He fake coughed and covered his mouth dramatically. "Blargh. Think I puked in my mouth a little there."

"He seemed a good sport about it."

"'Course he did. The guy's a goddamn rock with a personality to match." That gave Bruce pause. He looked back at his laptop, idle and waiting for his commands. He stared at the simulations for his project, his failed simulations because he couldn't find a powerful enough stabilizing agent. And then it occurred to him. But the idea was so crazy that he couldn't really digest it for a long moment, his eyes blank and his mind going a million miles a minute. "Oh, I know that look," Tony said. "That's the 'Eureka!' look. What is it?"

"A stabilizing agent," Bruce murmured. The corner of his mouth turned in a little smile. "The Holy Grail of stabilizing agents. The best one there is."

"Rogers?" Bruce nodded. Tony rolled his eyes. "You're dreaming now. Been there, done that. You know better than anyone that actually finding the Holy Grail would be easier than recreating the super soldier serum. Not to mention I'm pretty sure SHIELD would be on your ass faster than the Green Peace yuppies of the world would be on your super plants."

"Maybe I don't need to recreate it. Maybe I just need some of it. Steve's body makes its own supply. If I could just get a blood sample, just enough to try and confirm some of my hypotheses–"

Tony chuckled. "Good luck with that, Banner." At Bruce's downtrodden expression, he turned from his work again. His expression was a tad exasperated. "You know, I have this crazy idea. You could, I dunno, ask him. He's Captain America. Saving children and curing world hunger falls under the job description."

Bruce grimaced. His brain had been running of its own accord, charging through the known chemical properties of the super soldier serum and relating them to the known problems with Extremis. These two chemicals were veritably polar opposites. Equilibrium versus radical amounts of uncontrolled energy. Self-sustaining versus fast-burning. Both conferred enhanced metabolism, constitution, power, and regeneration, but they did so in completely different ways. Could they really complement each other? Could the addition of the serum stop Extremis from destroying his plants before they could really bloom?

It didn't matter. It was a moot point. "No," he said, shaking his head. "I couldn't do that. It's just… It's not right."

"What? Asking him or fooling around with the very same thing that gave birth to tall, green, and deadly?"

Bruce winced sheepishly. Tony knew him too well. "Asking him."

"Well, then, I guess if God intended the world to have an endless supply of tomatoes, he would have made tomato plants a little more durable." An uncomfortable moment of silence crept by. Truth be told, this wasn't the first time an idea like this had occurred to him. Ever since learning that Steve had been found in the ice, he'd been wondering about it. The scientist in him hungered for answers, for a chance to poke and prod and analyze Steve to figure out why Project: Rebirth had succeeded and his own experiments had so horrifically failed. Sometimes he'd downright obsessed over it when the pain and anger had been too great to ignore. Sometimes he hungered for a cure so acutely that it was all he could think about. How he would do it. How he could extract the serum from Steve's blood and refine it and maybe, just maybe, reproduce it. But he'd never been able to bring himself to pursue answers to his questions. Part of it he knew was because he was too ashamed to have to ask Steve for anything, let alone something so private as unfettered access to his DNA and his cellular biology and his body. But when it really came down to it, it was a selfish reason. He was terrified of what he would find.

And everybody else seemed to have let it go. Since saving Captain America, nobody had asked Bruce for his opinion on restarting Doctor Erskine's work. Nobody had posited that investigating the situation would be beneficial. Nobody good, anyway. Not SHIELD or the United States government or the Avengers or Steve himself. He was the only one who couldn't entirely let it be.

Tony's voice cut through his uncomfortable thoughts. "You know, just because it didn't work doesn't mean it wouldn't have worked." Bruce glared at Tony; did he actually think that thought had never occurred to him before? "Maybe you used too much Gamma radiation. Maybe you miscalculated something. Rogers had my dad and a whole arsenal of scientists behind him. Hell, if you had had me with you–"

"I know, Tony."

"Just sayin'."

"I know."

Tony pursed his lips and appraised Bruce like he was some sort of flaw in one of his suits. Then he set his tools down. "Cheer up, Banner. I'm sure you have tons more combinations of uselessness and futility to keep you busy until you grow a pair of balls big enough to ask."

Bruce shook his head. "I'll grow a pair when you do."

Tony smiled. Bruce felt better seeing it for some reason. For the same reason he always did. "Harsh."

Well, even if the super soldier serum would work, he would need a lot more of it than just a few drops of blood. It really wasn't practical. So he put it out of his mind and went back to his projections and Tony went back to his tinkering.