"The one that got away" by Katy Perry. This is set in an AU after Headhunters.

Kate Beckett is thinking in a constant stream of what ifs.

What if she'd told Castle she remembered? What if he hadn't left? What if she'd just told him how she felt? What if she'd opened that door in LA? What if her mom hadn't been shot?

And each one of them hurts more than the last, because, she realises, every last possibility she had been considering had something to do with Richard Castle. Even her mom's murder, which was the most private thing she had. He's in that. He's in all of it.

She can't bring herself to refer to him in past tense, because he's still there. He is alive, his heart is beating, but it's beating away from hers.

He just left. He left her, he left them.

( Beckett sees him packing the things up from her desk, pulling open the draw and scooping the contents that belong to him into his bag. They clatter inside with a broken symphony of things that don't matter to him any more. She fits into that catergory. She's resigned herself to that.

"Cas- Rick, what are you doing?"

"I'm leaving."

"Y-what?" she chokes on words, her brain clenching itself into a tight, unforgiving fist around that thought. He's leaving.

"Yeah, I have... enough research for the Nikki Heat books. I think I had enough research a while ago, actually." Castle's tone of voice is very deliberately, painfully pleasant, but she sees that look in his eyes, a glimpse of pure hurting that she knows is only the tip of the iceburg. Is that her fault? Did she do that to him?

He's like a wounded animal, when he gets that expression on his face. He lashes out at anyone who tries to help, refuses to be looked after, and all he seems to want to do is slink off to lick his wounds in private.

"You're just- you're going?" He wants to leave, and all she wants him to do is stay, stay forever. She doesn't care what she has to do to heal him, she'll quit her job, throw down her badge on the captain's desk, if only he'll stop looking at her like she's the one ending them. Beckett wants lazy mornings cooking breakfast together and being woken up in the middle of the night when one of their kids has a nightmare, and- Castle, Castle, you said always.)

In another life,

I would be your girl.

We'd keep all our promises;

Be us against the world.

Beckett knows that only one and a half of the what-ifs were unpreventable by her. Her mom's murder? She couldn't fix that. And she knows that somehow, somehow, Castle leaving the precinct was her fault, but how was she meant to fix it before it started if she never knew what she was doing wrong?

She knows now; he didn't tell her. She doesn't think he knows that she knows.

But she'd been lying awake in bed, re-playing the days before he started to pull away over and over in her head, and then it clicked. The coffee on her desk. Her interrogation. ... And I remember every second of it.

She should have told him, but she didn't know how.

But maybe if she'd acted on one of the other what-ifs, she wouldn't have needed to.

If she hadn't left the hotel room in LA, if she'd leaned forward, grabbed him by the collar and kissed him like she meant it, then Beckett had almost no doubt they would be together by now. She would have baulked at first, of course. She would have freaked. Gotten angry. Gone back home to Josh as soon as she could.

But then she would have broken up with him, and eventually, she and Rick would have made it right. More than right. Perfect. It had been a year since LA. God, a whole year, so many what ifs. They could be engaged. Hell, she could be pregnant with his kid. If only if only if only-

("Yeah, probably to the Hamptons. I'll finish this Nikki Heat book, wrap it up. I think there's... only so far I can go with Nikki and Rook." her eyes sting. She actually has to swallow, hard. His words flow through her like water and are threatening to stream out her eyes.

"I..." there are so many ways she could finish the sentence. I love you. I'll miss you. I love you. I want you to stay. I love you. I'm sorry. I want to try and fix this. No matter what she says, he's already gone. She's not going to baring her heart only to have him shoot another arrow through it. "I thought you'd stay around longer."

They pause, and the silence is heavy on her already weighted shoulders.

"So did I," he murmurs, soft, and for a moment there's a glimpse of the Castle she knows so well, only this is a Castle broken beyond any hope of repair.)

In another life,

I would make you stay,

So I won't have to say you were

The one that got away.

Beckett knows, looking back on it, that she should have tried harder, just a bit harder. She should have done something. Told him she loves him - because that is still very much present tense - given him a piece of truth to patch up the wounds her lies had inflicted on him.

But at the time, she hadn't known what had caused those wounds, just that they were there.

She had just thought that he hadn't cared.

Hell, she would willingly go back in time, get down on one knee and propose to him in the middle of the precinct if that would keep him there. Too late now. That's just another what if with infinite possible endings.

He has retired from his writing career.

The general public seems to believe it's because they broke up, which they did, but at the same time they didn't, because they never even got a chance to begin before it was all over. Beckett doesn't know what happened to his always. She thinks maybe his always expired the same time his I love you did.

Maybe he's the same as her, though, in that he doesn't appear to have a muse any more. If writing holds the same disinterest for him as her job now does for her, at least that brings her some sort of closure, that he's hurting as well. Because if they're equal, it takes the edge off the pain.

It's time to face the music, I'm no longer your muse.

She isn't chasing after anyone else. The phrase 'one-and-done' keeps echoing in the back of her mind, and she recognises that if he really was the one, she's done now. It's not really that bad. No one compares to Castle, anyway; no one ever has, for as long as she's known him.

She's going to spend every night for a long, long time sitting up and wondering.

In another life...