Outside, the New York City air in summer verged on the unbreathable, more like shower steam than oxygen. Inside Richard Castle's climate-controlled apartment, it was, always, an eminently civilised temperature. He was therefore perfectly comfortable in his old sweatpants and a t-shirt, leaning back in his writing chair with his laptop across his lap, bare feet up on the desk, enjoying a whisper of cool air playing over his toes.

His eyes kept straying to his phone, with her text still up: Are Alexis and Martha in the Hamptons all weekend?

What that meant, who knew. Castle sighed and forced his attention back to the screen. He was way behind on his deadline, suffering some form of blockage, though God alone knew why, since working with the Beckett provided more juicy, twisted, fascinating material than he, or even Patterson, could use in a lifetime. Gina hassled him for the first draft at least once a day, and the pressure was building to the point where Beckett had cornered him in the break room this afternoon and asked if he was okay. He'd admitted that he was finding it difficult to get into the headspace of the main suspect in the new Nikki book, and the manuscript had stalled as a result.

"Still? I thought you were having that problem a couple of weeks ago."

"The guy, Mark Cooper, is smart, and very, very dangerous. He's into intimidation, extortion, drug smuggling... Needless to say, he's left a trail of bodies in his wake, including a cop from Nikki's precinct. He's led Nikki on a wild goose chase all over the city for weeks, and she's just now caught up with him." He'd jammed a hand into his pocket and inspected the bottom of his coffee cup as though the answers were to be found there. "But I'm having a hard time putting myself in his place, you know? I can't make it real. Hence the book is now three weeks overdue."

Beckett – admittedly not a wholly impartial audience – had been eager to help push the new story into the light of day, and asked if there was anything she could do. He'd been quite proud of himself, giving neither the comedy nor the flirty reply, but showed his appreciation for the offer by making her an exceptionally good cup of coffee, allowing her neural transmitters to fire away, even if his couldn't.

"So what would help you get into that headspace, Castle? Want me to drag you into interrogation and make you feel like a bad guy?"

Even before he could blink his shock that she would so much as joke about that, a much more primal reaction pulsed through him. "You'd love that, wouldn't you, Beckett?" And so would Ryan and Esposito, who would undoubtedly be on the other side of the mirror simultaneously watching, filming, tweeting, and holding up scorecards. It would be on the internet before he'd got out of the chair. Still, if his career was going to go down in a blaze of shame, he couldn't think of a more pleasurable way to go. Kinky, yes, but he'd more than made peace with his quirks. He wondered if Beckett had made peace with hers.

She'd given him a look that seemed half curious and half... appraising? She disappeared back to her desk, leaving him staring after her with images of himself at her mercy in interrogation. That hadn't helped his concentration, and he'd had to go out for a long walk.

He sighed and rubbed his eyes. The clock on the computer read 11.15pm. He'd been sitting here for four hours and hadn't written a thing. He jumped at the sudden banging on the door.

"NYPD, open up!"

More banging and yelling as he ran to the door. That sounded like... He yanked the door open. "Beckett?" She was in full SWAT gear, pointing a gun right at his chest. Reflexively, his hands went into the air and he fell back a few paces. "Beckett?" he repeated, searching her face for some clue as to what the fucking hell was going on.

"Detective Nikki Heat, NYPD, against the wall! Now!"

Nikki Heat? When he continued to stare at her, slack jawed, she barked, "Now!", and gestured with the Glock 9. Nothing in her expression gave him to understand that she was in any way kidding, but he could see the ammo clip had been removed.

He blinked. Slowly the light was dawning. He managed to stop a huge grin. Instead, he nodded and moved to obey. The moment his hands met the cool surface of the wall, she began patting him down. Her hands roamed briskly, efficiently, and none too gently from his hair, down to his chest, over his hips, into his pockets, up his inside leg, and into his groin, giving a squeeze for good measure. Castle closed his eyes and swallowed hard.

"Put your hands behind your back."

He did. And felt another pulse go directly to the good place.

There was the unmistakable sound of a clip being undone, and a moment later she'd taken his hands in hers and unforgiving steel bracelets ratcheted into place. It wasn't painful, but it wasn't comfortable, either.

"Turn to face me."

A small warning light came on in his brain. This was great and all - really great, but did she really feel up to this? He didn't want her to feel pressured in any way, especially when it came to something like this. He levelled his gaze at her, himself again. "Detective," he said carefully, deliberately leaving it ambiguous as to whether he was addressing Heat or Beckett, "are you sure you want to be doing this? Are you sure this is... okay?"

She did meet his eyes for a beat, mostly, though not entirely, unreadable. Her voice warmed a degree. "Believe me, there's nothing I'd rather be doing."

He nodded. Okay, then.

Her eyes and voice hardened again. "Mark Cooper, you are under arrest on suspicion of extortion, drug smuggling, and the death of a police officer. You have the right to remain silent. Anything you say can and will be used against you in a court of law. You have the right to talk to a lawyer and have him present with you while you are being questioned. If you cannot afford to hire a lawyer, one will be appointed to represent you before you answer any questions. Do you understand these rights as I have read them to you?"

Still in shock, he nodded. How would Mark Cooper be reacting to all this? Castle had no idea, but that was fine; she was being authentic enough for the both of them.

"Yes or no, Mr Cooper?"

"Yes." He watched her remove a plastic bag from a pouch on her belt.

"Open your mouth."

"You want me to..." That made a change.

"Yes. Open."

He obeyed, allowing her to swab the inside of his mouth with a long Q-tip.

The swab went into a container. "This will be added to the DNA database at the National Crime Information Center. It will remain there whether or not you are found guilty on this occasion. Do you understand?"

He nodded, and then remembered to say, "Yes." God, this was so hot! DNA samples! His inner geek did a dance of joy.

Her eyes met his again, absolutely unwavering. "Owing to the nature of your past crimes, there's another sample I'm going to take from you."

His mouth fell open. "Beck – Nik – Detective..."

"Get on the couch. Face down."

She didn't have to tell him twice, but he hesitated anyway, pushing the boundaries just for fun.

She reached for her radio. "Are you going to follow my instructions willingly, or shall I call someone to assist you?"

"No." It felt as though the temperature control in the room had failed. He moved over to the couch and stopped. "Ah, not the angora."

She grabbed the blanket and threw it onto the armchair. She trained the gun on him again, and walked backwards to the linen closet, where she took out a couple of old camping towels. She returned to the couch and spread the towels over it.

She met his eyes. "Face down."

He obeyed with some difficulty, between having his hands cuffed behind his back and the fact that he was harder than he'd ever been in his life.

"Lift your hips."

Again with difficulty, he did his best. As soon as his hips were clear of the cushions her cool hands took control of his sweatpants and boxers, drawing them halfway down his thighs. He heard her breathing deepen as another packet was opened. He couldn't see what she was doing, but he heard the sound of a hand going into a latex glove, a sound he'd heard at every crime scene they'd been to. He was never going to be able to hear that noise again without thinking of this. Forget that, he was never going to sit on this couch again without thinking of this.

Her hand rested on his shoulder for a moment, and her voice softened. "Is there anything I should know before proceeding?"

"No," he managed. "It's fine." New, but fine. Fine in a brain-exploding kind of way.

There were some soft, opening-things kinds of noises, and then her cool, gloved hand spreading him open. Face down, handcuffed, utterly vulnerable, he held his breath and tried not to come instantly as her insistent, slippery finger entered him and slowly but inexorably pushed deep, only to withdraw and slide in again.

That was it. He jerked helplessly against the couch, desperately seeing leverage and friction for his cock as she hit the spot with gratifying accuracy. He was lost. "Beckett!" he gasped, and came, the world whiting out.

He lay there, warm, wet heat spreading beneath him. Dimly he heard the gloves come off, and then her hands were on his, manipulating the cuffs until they came away and his arms were free, pulling a groan from him as he let them drop to his sides. A moment later, her warm, soft hand pushed up under his t-shirt and gently stroked his back while the spasms subsided. He kept his eyes closed. Now that the sex goggles were off, could he face her after this? Could she face him? Had this ruined everything?

He patted around until he found her hand. "Beckett? That was... I don't even have words for what that was. Thank you."

She squeezed his hand. "Anytime, Castle. Don't suppose it helped you with Nikki Heat, though."

He turned so that he was on his side, facing her, letting his thumb play over her knuckles. "Well, actually it did. I did get some idea of how it feels to be a suspect, though I'm not sure how regulation your procedure was on this occasion."

She grinned. "Not very."

"Up to the DNA part, though..."

"That was accurate."

He nodded. And had a thought. He eyed her carefully. "Have you ever been on the wrong side of an investigation?"

End.