A/N: The song this story is based around had been stuck in my head for a while now. I've had the lyrics open in a tab for at least two weeks, waiting for inspiration to strike. And it did, at just about 12:30am. I'm not telling what song it is until the end - if you can guess, all the more power to you.

Disclaimer: Castle, in all its glory, belongs to Andrew Marlowe and crew.

"Don't leave."

She paused, her hand on the doorframe. "I have to. I can't… Not anymore."

His voice was perilously close to begging when he took the next step forward, wanting to touch her but afraid that it would just drive her away faster. "Kate. Please."

Don't look back, she told herself. You look back and you're done for. It killed her, even through the anger coursing through her body, to just walk out into the hallway and leave him standing in the living room. She didn't have to look to know just how he looked at that moment when she disappeared around the corner. That was part of the reason she was leaving in the first place; he had gotten under her skin, dug his way in there and made a happy home right next to her heart where no one ever touched. She had tried to evict him and each time, he found his way deeper into the organ, spreading through her veins until she had been sure she wouldn't be able to live another day without seeing him in the morning, waking up with his face next to hers, snoring softly with the warm puffs of breath ruffling her hair or falling asleep next to him when her body finally gave out from exhaustion after running on fumes of caffeine.

The other reason was simpler to explain, less emotional. She had seen the board the few times before that night while wandering around at ungodly hours when her mind refused to shut down to sleep. She never turned it on, happy to let the plot of his next novel stay a secret until she held the published hardcover in her hands. Tonight had been different and she had no one to blame for it.

.

The case had closed faster than she thought it was going to. It had looked tricky; hints of the mob tangling up with love made for something that looked vaguely like West Side Story. Castle had said as much as he quoted the musical at every chance, claiming that was what happened when you were raised by a Broadway diva at her career prime.

Kate had sent him home for the night, promising she'd be back by nine o'clock if the case didn't close itself earlier. And he had left, assuring her that dinner would be waiting for her, either on the table or in the fridge. She expected to come home to a dark apartment, nuke the leftovers, and crawl into bed next to him to sleep for a few hours.

What had happened was out of left field. The killer, a young boy no older than high school age, strolled into the precinct and confessed. Signed his confession, let Ryan bring him down to Holding for the night before he would head to the Tombs to await arraignment.

So Kate had rolled out of the precinct not an hour after Castle had hailed a taxi back to the apartment. She was pleased with herself. She's surprise Castle, be able to eat dinner with him, then fall asleep with him at the same time instead of joining him hours after he had drifted off.

The loft was eerily quiet when she unlocked the door using the shiny silver key Castle had handed her two weeks ago "just in case." There was a glow from the kitchen from the fireplace, another across the room from his study. Normally the man needed bright chaos to write, so the near-darkness was confusing. She dropped her purse and jacket on the back of one of the dining table chairs, tiptoeing toward the study to open the door.

And stopped short.

"What?" Her voice was barely a whisper, shock taking away any urge to scream.

He turned from the SmartBoard, his face dropping instantly. "Kate. This isn't…"

Anger had started creeping in, a welcome rush. "Do not say 'what it looks like,' Castle. I'm not stupid." She pivoted, going to get her things from the dining room.

"I know you aren't." He ran after her and managed to grab her wrist. "Kate, let me explain."

"I don't want to hear it, Castle!" she yelled, trying to pull her wrist from his hand and failing. "How could you?"

"For you, Kate. Always for you. I had to."

Kate jerked her hand away from his, snatching up her jacket and purse. "Bullshit, Castle. You did it for you, just like you did three years ago."

.

In the elevator, she let her head fall back against the wall with a dull thud. He hadn't followed her unless he was jogging down the stairs to intercept her in the lobby. He wouldn't make a scene there, not with Eduardo sitting at the desk with the latest Time and a cup of coffee. Knowing him, he'd show up at her front door in an hour or so with flowers or chocolate or wine to smooth things over.

But gifts wouldn't heal this hurt, not when the betrayal went this deep.

Kate made it to her own lobby before her resolve started to break. She wanted to turn around, grab that cabbie that she had just paid, and get back to Castle. Apologize for her behavior and let him explain. Then she remembered what she had seen, what he had been doing and kept walking up the stairs of her building.

It didn't take her long to pack – she had always traveled light. A quick voice mail was left for Gates at the precinct, telling her she was taking a week of vacation time. A mass text was sent off to her team and Lanie. Then Kate locked up the apartment and went to find another cab in the night to get to JFK.

He was still standing in the living room, in the same place she had left him. That was where Alexis found him as she came downstairs for a glass of water.

"Dad?" Her voice was thick with sleep, but the concern was obvious. "Where's Kate?"

He wasn't sure his own vocal chords would cooperate with him, but he tried. "Gone. She's gone."

A/N: I promised I'd put a story out before Monday. There will be more because I have the rest of this story completely planned out to a total of four chapters. And I know exactly what will go on in each of those chapters as long as the characters behave and go along with me.

In other news, Logan broke 25K on NaNoWriMo last night - 25,040 to be exact, but who's counting? Halfway done!

Reviews would be fantastic. Really. Take a break from fangirling/crying over the Kill Shot promos and drop a few words here. I'd be eternally grateful for something to read before I go out on tour tomorrow. (And I'm spoiler-free, so don't go rambling about the promos since I will ignore the messages until after Monday's episode. Just sayin'...)