A/N: Alright, I was going to write one long chapter, but I really wanted to get something up before Christmas for you guys. So, I've split what I wanted to write into two parts. Merry Christmas!!

"Her what?!" Flack asked in complete shock. "Stella was engaged?"

"She and Nate were engaged for nearly two years," Mac nodded.

"How is this the first I'm hearing of him?" Flack asked in confusion. "Did it end badly?"

"I don't think broken engagements ever end well," Mac said. "But given the circumstances, no, I'd say it ended about as well as any relationship could. I suppose it helped that it didn't have anything to do with them not loving each other."

"Isn't that all that matters?" Flack asked.

Mac smiled sadly and shook his head at the younger detective. "I wish, Don, believe me, I wish it were that simple. When you've lost someone you love, it can be hard to see how you could need anything else, but sometimes, circumstances just get in the way, no matter how strong the love is…and what Stella and Nate had, it was pretty strong."

"So what happened?" Flack asked.

"Did you know that our Arson Explosives Squad used to be the best in the country?" Mac asked. "They had a ninety-two percent solve rate, and an eight-seven percent conviction rate – unheard of in law enforcement, as you know."

"What does that have to do with anything?" Flack asked in confusion.

"If you had a case today that involved an explosion, or a major fire, who would you call for help?" Mac asked.

"The feds," Flack said very matter-of-factly, pausing as he thought about his answer. "Oh, got it. Stella and this Callahan guy were on the AES when it was the best, and now it's not so great. I guess their breakup had something to do with the unit falling apart?"

"More like the other way around," Mac said. "The unit's destruction had everything to do with their breakup."

"How's that?" Flack asked.

"The AES had a reputation for being the best, but they also had a reputation for not always doing things, well, strictly by the book," Mac said. "When Giuliani was in office, it didn't matter – as long as the results came through, the administration didn't ask too many questions. After he left office, though…"

"The old changing of the guard," Flack said knowingly. "Yeah, I lived through that too, Mac. The new administration forced out every department head hired under Giuliani…well, everyone except you, I guess."

"Mike Rodriguez at the AES was the first to go," Mac said. "He was an easy target – truth was, he'd gotten away with more than anyone should have, and he'd done it for years. Problem was, he wasn't just the head of that unit, he was it's heart. Without him – and with the rules suddenly being strictly enforced, and every little violation getting written up – morale dropped and the unit started hemorrhaging good officers."

"The brass just let them transfer out?" Flack asked.

"Not surprisingly, no," Mac said with a dry smirk. "They thought if they blocked transfers, the unit would pick itself up and start functioning again. I guess they didn't count on the fact that this wasn't just another group of officers – it was a group of seasoned investigators who other agencies would snap up in a heartbeat. When word got out that the NYPD's AES had more than a few unhappy officers, you should have seen the way the headhunters started circling. Rodriguez left in January – by the fourth of July, almost a third of his officers were gone. By Labor Day, it was more than half."

"Including Nate Callahan," Flack guessed.

"Actually, no," Mac sighed, shaking his head with what Flack almost interpreted as regret. "Stella loved the NYPD; she couldn't see herself working anywhere else. When it became clear that things weren't going to get better at the AES, she came to me for help – asked me to pull some strings, get transfers for her and for Nate to other departments."

"And you got her the transfer to the Crime Lab," Flack filled in.

"It wasn't terribly hard, she was already qualified, and she passed the detective's exam with flying colors," Mac said. "I had to strong-arm my way past the bureaucratic red tape that was holding everyone in at AES, but once I did that, it was easy."

"And Callahan?" Flack asked.

"I could have tried harder," Mac admitted. "I should have tried harder, to be honest. You know how I am with playing politics, though. I suppose at that point, I was a little bit like you – I thought that if they loved each other, they'd find a way to make it work, with or without his transfer. So I made a few calls, but nothing like I could have. And when it didn't come through, I didn't fight for it. I had what I wanted – Stella was staying – so I didn't even think about what it would mean for them as a couple. And he tried, I've got to hand it to the guy, he really tried. He stayed at the AES probably six months after that, putting up with all sorts of crap and abuse from the brass. Eventually, though, he just couldn't do it any more, and he took the job with the FBI."

"And that was the end of the engagement," Flack said.

"Well, the beginning of the end, anyway," Mac said. "She's never told me this, but I know Stella almost left when Nate did – I have a friend in human resources at the Bureau who told me the paperwork had all gone through for her to have a permanent posting in D.C. with Nate, but she pulled the plug at the last minute."

"Why would she do that?" Flack asked.

"To be honest, I think he talked her out of it," Mac said. "And I have to respect him for that. Stella loves New York, it's her home – he knew that better than anyone, and he knew she'd never have really been happy in D.C. She would have put up with it to be with him, but she wouldn't have been happy. So he convinced her they could make the long-distance thing work, but as it turned out, they couldn't. They tried for almost a year, but in the end, it was just too much."

"How did I not know this?" Flack asked. "When did all this happen?"

"You know how private Stella is," Mac said. "She hardly talked to me about it, let alone anyone else. I think she ended it sometime in June of 2004, so that would have been right before you started working with us, I suppose."

"Wow," Flack muttered. "How long were they together?"

"Almost five years," Mac said.

"Five years," Flack repeated with a shake of his head. "Five years and then nothing. That's got to hurt."

"I wouldn't exactly say nothing," Mac said cautiously.

"What do you mean?" Flack asked in confusion.

"Just because they ended the engagement doesn't mean they stopped loving each other," Mac said. "And it certainly doesn't mean that was they never spoke again. They've both had relationships since then, but they always seem to come back to each other when they need someone."

"Like when?" Flack asked.

"Well, after Frankie, for instance," Mac said, drawing a confused stare from Mac. "I know, everyone thinks she stayed with me. Truth is, I offered, but she turned me down. Said she needed to stay at her own place, but when I went by in the morning, she'd packed a bag and was gone. She never said anything for sure, but I put in a call to my HR friend at the FBI, and she told me that that same night, Nate Callahan put in a last minute request for a week of personal necessity time. Same thing after the fire at her apartment, the whole fiasco with Drew Bedford, Aiden's death…and it's not just on her side, either – Nate's had a few scares over the years, and not-so-coincidentally, they all seem to line up perfectly with Stella's personal necessity time or her last-minute sick days."

"So why isn't he here now?" Flack asked.

"I don't know," Mac sighed. "Maybe he doesn't know."

"It's not like we kept her identity a secret," Flack pointed out.

"No, but her condition isn't public knowledge," Mac said. "Just the fact that she was shot. I think – and I don't know for sure – but I think they have an arrangement, to let each other live their own lives, unless he calls her to come or she calls him. If he doesn't know that she can't call…"

"Would she call him?" Flack asked. "Would she want him here?"

"Probably," Mac said reluctantly. "Yeah, she probably would."

"Then you need to call him," Flack said.

"Me?" Mac asked in surprise.

"Well, I don't know him, you do," Flack pointed out, pausing as his phone began vibrating in his pocket. Pulling it out and flipping it open, he frowned as he read the text message.

"Everything alright?" Mac asked.

"It's the Harrison case," Flack said, tossing the phone back into his pocket and standing up to leave.

"That's Lindsay's brother-in-law, right?" Mac asked, feeling just a touch out of the loop after having shifted so much of the energy that was usually focused on work over to Stella.

"Yeah, her sister thinks she remembered something that might help, so I gotta go," Flack said. "Mac, I'm serious, though. If what you told me is true, you need to call the guy. If nothing else, he's someone she'll actually remember."