Not my characters. Spoilers.
The characteristic thrum and wheeze of a Time Lord's conveyance filled Odin All-Father's throne room. At first the guards drew their weapons and made ready to attack, but Odin stopped them with a wave of his hand. "It is the blue box of the Doctor, our friend. At ease."
He had first made the Doctor's acquaintance during the war with the Frost Giants. At the time he had curled brown hair and a long, multicolored strip of cloth about his neck which swung and trailed as he walked. His advice about ending the war quickly had saved Asgard much bloodshed.
The second time he visited he had changed his face, in the manner of Gallifreyans who avoided death for another round, and had golden hair and a coat the color of wheat. He had come to announce the tragic news of the destruction of Traken and relate the tale of how it had come about, presenting also Nyssa, a princess of Traken who had been the sole survivor. She had been given warm welcome and counsel in her time of mourning. During the visit he had played also with Odin's young sons, teaching Thor a game known as "cricket" with a large wooden club that struck a thrown red ball, and enthralling Loki with tales of time and space.
The third time his face had changed yet again - he explained that for himself it had been many centuries since his last visit - and his coat was greener than emeralds, his form small and delicate. He brought sad tidings of the Time War and the possibility of the Time Lords losing their honor and nobility in their desperation to overcome their foes. So he had stolen the Tesseract, a source of power, that he feared might be misused to harm innocents. He begged Odin to take the Tesseract and protect it, even to the extent of making others believe it was relic of Asgard rather than Gallifrey. Odin swore to do as he asked.
The fourth time he had almost crashed into a pillar, such was the state of his smoking and battered ship. Out had collapsed another face, hair cropped close to his skull and a short coat of leather. It had been days before anyone, no matter how gentle and caring, had managed to coax him into speaking of Gallifrey's destruction.
This time the Doctor wore a long coat of brown and hair that stood nearly on end. "Hullo, Odin. I'm afraid I can't stay; I have been summoned to the Oodsphere and I have delayed too long already. But it occurred to me that you've been a good friend and ally all these years, and I should pay you back for that."
"It was you who first allied with me, honorable Doctor," Odin replied.
The Doctor smiled, but there was sadness in his eyes. He walked up to the throne, giving a small bow, and handed Odin a silver device with a single button. "If you need my help, press this and hold down until a count of ten. I will come. I can't guarantee that it will catch me at the correct time, but I will know it as something I built and will trust you even if I will not have met you yet. Farewell."
"Farewell, if you truly cannot stay even for a few hours."
The Doctor shook his head. "If I don't go now I never will. And I would rather the four knocks didn't sound here. By the way, have you told Loki yet?"
Odin didn't bother to ask how the Doctor knew, for the Time Lord worked in mysterious ways. "He is not ready."
"You shouldn't put that off too long. Free advice. Goodbye and good luck, All-Father." And he shut the door behind him, his box fading away.
Half the SHIELD personnel, including Natasha Romanoff, had their guns trained on what looked like a blue phone booth that had noisily appeared out of nowhere in one of the kitchens on the airship by the time its door opened. What exited was somewhat anticlimactic - a gawky young-looking man with floppy hair, a gray coat and a bow tie. He had his hands up. "Ooh, dear, a lot of guns. Come in peace, et cetera. Are Thor and Loki on board? Tell them their dad sent an old friend to see how they are."
"So you're from Asgard too?" Natasha asked.
He laughed. "Do I look like it? No. But my people were allied with them."
"They're gone now. Long story. I'm the Doctor. Hello. Could we be putting guns down now? I'm unarmed."
That's when Thor barreled through the crowd of agents and wrapped the Doctor in a hug. "It is good to see you, Gallifreyan healer!"
"Oof! Yes, it's good to see you too...not so tight please..."
"Sorry, sorry." Thor let him go and adjusted his bow tie for him with large hands.
Natasha gave a few quick orders, sending the other agents away. "Thor's vouching for you, but I'll have to ask that you come see Fury."
"Oh, naturally, naturally," the Doctor replied, all amiable, despite his slight wince from Thor's arm around his shoulders in heavy chumminess. They followed Natasha through the halls to the elevator. "My, how you've grown. Odin's worried about the two of you and whether you'll be able to return, Thor. I'm the only person he knows who could easily take you back home, so he summoned me. How is Loki? I've heard about some of the - well, it's unfortunate, isn't it? Such a sweet child he was."
"Perhaps you can coax him, Doctor. He always thought highly of you. We have him in a cage at the moment."
The Doctor raised his eyebrows. "That bad? My."
"We do seem to be having an epidemic of aliens just popping out of nowhere," Tony Stark mused dryly as the Avengers present on the ship watched the video feed of the Doctor being interrogated by Nick Fury. At least Fury seemed to think it was an interrogation. The Doctor seemed to think it was some kind of cheerful social call, at once point even producing a packet of cookies from his pocket and offering "a nice ginger biscuit", eating them himself when the offer was rejected.
"The Doctor is a good man," Thor attested yet again.
Steve looked up from where he was poking, bewildered, at an iPad Tony had deliberately left on the table to see how the temporally displaced man would react. "You said he was from a place called 'Gallifrey'. Where is that? What's it like?"
"It was the land of the Time Lords, the oldest and wisest race of them all, even more so than my people. They learned the secrets of time travel, so ingrained in their very bones that they may even cheat death by changing their bodies and make themselves into new men, yet still remembering who they are. They fought a terrible war. He is the last of his kind." Thor scratched his beard thoughtfully. "He seemed like a kind of wizard or Elf when I was a child, appearing out of nowhere unexpectedly with gifts and tales to tell."
"How old is he?" Natasha asked.
"Father said nearly a thousand years."
That's when Bruce emerged from the lab. "I've been scanning the Doctor remotely using some of your tech, Tony. He's definitely either an alien or a surprisingly well-adjusted congenital birth defect - two hearts."
Fortunately for everyone, especially Fury's sanity, the Doctor's suggestion of calling the UK and an organization known as UNIT confirmed the Doctor's status as a friendly alien.
"You really should let me see Loki," the Doctor said once he was told of this.
"Sell me on it," Fury replied.
"First, Loki unfortunately is very upset at his brother right now and considers humans to be insects - very wrong of him, by the way, humans are delightful, always surprising me - but he's never been anything but fond of me. Second, I know you want to know where the Tesseract is. So do I. And while I try not to make a habit of it, and I really don't like doing it to someone unwilling, my ability to read minds is preferable to torture. More accurate, as well. It unfortunately requires physical contact."
A short while later Thor and the Doctor were granted access into Loki's cell. "Father has sent the Doctor to assist us," Thor explained.
Loki raised an eyebrow. "I see that you have died again, Doctor. Come to mend my tortured soul?"
"That would be preferable to the other options, yes," the Doctor replied, pacing. "Had a friend like you once. Brilliant, absolutely brilliant - but as a result of being used as a pawn in a larger scheme went completely mad. We spent centuries fighting across the cosmos. I am fairly certain he's dead, but then again I was before and was wrong before. The point is that I'm tired of losing people I like. And I liked you very much. Boundless potential. Don't throw it away."
"If you have nothing further to say than such triteness..."
The Doctor shrugged. "I like giving people a chance. Some even take it. Thor, hold him down."
Thor promptly shoved Loki to the floor, then placed Mjolnir on his chest to act as a Loki-weight. Loki laughed even through his discomfort. "So the true Doctor comes out. Tell me, as I have always wondered, is the reason you were the sole child of Gallifrey to survive the Time War that you were the one who destroyed both sides?"
"To save the Universe, yes, and if you plan on goading me with that be aware that there is no level of guilt, regret, or sadness that anyone could put me through that I haven't put myself through already." The Doctor sat cross-legged on the floor just above Loki's head. "You are welcome to verbally tell me the location of the Tesseract. Taking it directly from your unwilling mind will be painful for both of us."
"Let there be pain."
"You petulant child, you are being offered a chance!" Thor was near bellowing in frustration.
"And how much of a threat did I have to become before you felt such kindness and empathy towards me?" Loki asked.
"If you want to help, Thor, hold Loki's hand. He'll need it in a few seconds," the Doctor said. Then he placed his hands on Loki's temples.
"Don't touch -" Loki didn't manage to finish snarling at Thor before both he and the Doctor started screaming.
The Doctor wrote down a set of coordinates and a few additional notes before collapsing into a chair in the main meeting room. Agent Coulson gave him a pat on the shoulder and carried the notes off, presumably to confer with the rest of SHIELD. "If someone could get me an ice pack, please," The Doctor whimpered.
Everyone could see Loki in the cage in a huddled mass, staring at nothing with his brilliant emerald eyes. Thor was still beside him, holding Mjolnir in his lap and gazing at his brother in worry.
"Would you like some aspirin, too?" Steve asked quietly.
"No, aspirin would kill me. Cup of tea would be lovely, though, if you happened to have any on board." When he noticed the stares, the Doctor added, "I spend most of my time on Earth in the U.K. Though I did travel around America for three months in 1969, about fifty years ago from my perspective."
Steve nodded and headed for a kitchen.
"Why does your time machine..." Bruce began.
"Time and space," the Doctor corrected.
"Why does your ship look like a blue phone booth?"
"She has a camouflage protocol that broke when I was in 1960's London," the Doctor replied. "Oh, sorry to correct you; I can be pedantic about her sometimes. It occurred to me that you are someone I wouldn't want to anger."
Bruce laughed ruefully. "I see word gets around. When I'm angry things get broken."
"Oh, it's not as bad as it could be. When I'm angry species cease to exist." The Doctor fumbled in his pockets and produced a handful of hard candies in crinkly wrappers. "Ooh, snozzberry flavor. Anyone want one? They're my favorite treat from Yngvi in the forty-second century."
Tony took one of the candies and peered at it. "Isn't it uncomfortable flying around in such a little box?"
"The TARDIS is bigger on the inside," the Doctor replied. "Would you like a look? I haven't shown her off to any humans for a while."
That's when Steve returned with an ice pack. "No tea, sorry."
"It's alright, the TARDIS probably has a cuppa waiting - she's considerate. In fact, why don't we all have tea on the TARDIS? It's teatime somewhere. Natasha's welcome too; if someone could call her." And the Doctor led the way.
"To put it as elegantly as I can," Tony said the moment they stepped across the TARDIS threshold, "this is the shit."
Bruce eyed the console and its plethora of weird odds and ends. "I take it you've repaired it a lot."
Steve just stared with his mouth open.
The Doctor grinned. "It's actually meant for six people to fly, so I've had to make adjustments."
When Natasha followed them, the ship immediately hummed and the lights flickered. "What's that about?"
The Doctor's smile became sad. "I think you remind her of someone she misses. I pick people up to travel with me sometimes, you see. The intention always is just to sightsee, but you know how it is - dictatorships need toppling, genocide needs averting, the fabric of the time-space continuum needs someone to stitch it back together. Humans in particular always seem to be getting into trouble."
"Is the TARDIS run by an A.I.?" Tony asked. "Yours must be very advanced."
"Oh, there's nothing artificial about my Sexy. TARDIS'es are grown, not built. They're a species that developed a symbiotic relationship with Time Lords." The Doctor stroked a wall panel and the ship purred. "Now, now, dear, they're just having a short visit."
"How big is this place?" Steve asked, having finally found his voice.
"Frankly I've no idea. Always finding new rooms. We had to get rid of the swimming pool and bowling alley, though, in a recent misadventure. Once a passenger got lost in the wardrobe for five hours." The Doctor licked his finger and raised it towards the ceiling, as if testing a nonexistent breeze. "And...there's definitely a tea set in sitting room two, though I have to warn you the scones might be a little stale."
"Doctor, this is lovely but..." Natasha began.
The Doctor held up a hand. "It'll good to have some fuel in us before we go rescue your friend Clint and the rest of the people Loki's bewitched. Making them themselves again is going to make my current headache seem like a little twinge, and if I'm going to make it through today I need at least a whole pot of tea for myself."
The Doctor took Steve aside for a moment after everyone else was situated around the tea table. "You're probably wondering if I can take you back to your own time."
"Well, yes." Steve looked at the alien hopefully.
"And I have to tell you that I can't. Because history says you never went back to your own time, and it's the kind of history I cannot rewrite. I'm sorry."
Steve took at deep breath and nodded. "Thanks for being honest with me, Doctor."
"Of course. Have you tried the sticky buns?"
The Doctor expelled Tony from the TARDIS after he caught him poking at the console's innards. "But it's so cool! Some of it's mechanical, some organic, and I think I may have grasped..."
Five minutes after the SHIELD personnel noticed that their Avengers - minus Thor - had been whisked away, the Doctor brought them right back. Plus Clint Barton and everyone else whom Loki had pressed into his service, all a bit discombobulated but in one piece. Steve and Natasha emerged soon after. Bruce had to be dragged out.
"Honestly, you're almost as bad as Tony," the Doctor grumbled.
"All right, I will admit that you have proven a help," Fury told the Doctor.
The Doctor smiled. "Good. Because I'll be taking the Tesseract, Thor, and Loki with me, and I don't want you to make a fuss about it."
"Returning them to Asgard, I assume."
"Thor, yes. The Tesseract I'm keeping to help power the TARDIS so I don't have to keep hunting for interdimensional rifts for refueling all the time; and because it originally came from Gallifrey, making me the rightful owner. Not to throw my weight around too much, haha. And my deal with Odin was in exchange for fetching Thor back safely, I would return Loki eventually. I have seen Asgardian justice and it is not what would help Loki at this time."
"You think you can fix him somehow?"
The Doctor tapped the side of his own nose. "Trust me, I'm the Doctor."
"This ship is a wonder of wonders! I will miss you, Doctor. You should visit more often."
"I'm sure I will, Thor. Good luck."
Thor did his best to hug the Doctor gently. and there was a catch in his voice. "Take care of him."
Loki did not emerge from the wardrobe until he was certain Thor had left. The Doctor had locked a bracelet around one wrist that let him roam freely about the TARDIS but only within ten feet of the Doctor if outside its confines. He had been directed to find some comfortable clothing of his choosing; his armor and helmet had been hidden away in some secret vault. He'd settled on black trousers, a buttoned green shirt, and a black coat, with boots of dark calfskin and a white scarf.
"What is it you plan to do with me?" Loki asked.
"I was thinking of having dinner with you, to start with," the Doctor said. "Found a room you liked?"
"Why are you doing this?"
That sad smile again. Loki didn't know how he felt about it. "Loki, Loki, Loki. The day that you don't need to ask that question is the day I can let you go."