Disclaimer: The only part of Castle that I own is the TV on which I watch the show.

Detective Katherine Beckett is at her desk, sneaking looks into the break room and coming to a rapid boil. No time for simmering, straight to 212 degrees Fahrenheit. She's sizing up the precinct visitor. Who the hell does she think she is? Who the fucking hell does she think she is, that she can come waltzing back in here? Not waltzing, oozing. Oozing and slithering in here in that satin dress that's so tight you can almost count her pu—. Nope. Not saying it. Not thinking it. Every man within eyeshot is, though. Thinking a lot more than that, too. A lot more ultra NC-17 thoughts than that. Slithering and oozing. Sloozing. That's it, she's sloozing. Total slut.

Serena Kaye. Talk about an inapt name. The woman is about as serene as a steel trap that can take your leg off in one snap. And Kaye? Sounds like the second half of okay, which she definitively is not. It's not remotely okay that Serena Kaye is standing here in the Twelfth Precinct, her hip thrust seductively against the counter in the break room as she chats with Castle. Flirts with Castle. And by the way, her nipples are practically piercing the top of that liquesecent excuse of a dress. The cleaning crew is going to be doing a lot of heavy mopping around here tonight, getting all the drool off the floor.

Beckett is going to do something about it. Now. Maybe she'll just blind the woman by flashing her however-many-carats it is engagement ring at her. How many carats is it, anyway? She hasn't asked Castle. Too embarrassing to contemplate, really, the money he must have spent. No one knows they're engaged. No one knows they're even dating. They've been together for seventeen weeks. Seventeen weeks and three days. And, wait, it was almost ten o'clock when she knocked on his door, so—and fourteen hours. Seventeen weeks, three days and fourteen hours. God, she's like a high school freshman with her first boyfriend, except that she had never acted this lovesick in ninth grade or at any other time. Ever.

He'd proposed to her halfway through her suspension, and she'd said yes. I will Yes. And they'd gone off to a mountain cabin that he'd found on some high-end hideaway site and disappeared for two weeks. And then they'd come back to the city, and she'd returned to the precinct and so had he, pretending not to be a couple. No one has guessed, not even Lanie. And judging from the tableau vivant in the break room, Castle is very successfully projecting "Unattached Male" like a neon "Rooms Available" sign at a motel. Beckett remembers exactly how smitten he was with Serena last fall, draping himself over her the way that dress is draped over her now. She shakes her head, hard, trying to repel a year-old image of him and Serena making out in a hotel hallway. Okay, okay, so she and Castle weren't together then. Doesn't make the picture any prettier.

The engagement ring, which for the moment she wears only at home, is on a chain around her neck. But in ten seconds flat it can be on her left hand; ten seconds more and that hand can be two inches away from Serena Not O'Kaye's face. Beckett doesn't even care that by brandishing her ring at the insurance investigator she'd be making public her relationship with Castle.

"Detective Beckett?"

In her peripheral vision, she notices the Captain poking her head out of her office. Shit. Shit. Shit. "Yes, sir?"

"May I have a word?"

"Yes, sir." Oh, Gates wants a lot more than a word. Or a sentence, or a paragraph. When she asks it like that, in a tone that's a dangerous mix of honey and hydrochloric acid, it resonates with all kinds of possibilities, none of them appealing. Beckett pushes her chair away from her desk and walks to the office.

"Have a seat, Detective."

"Thank you, sir."

"But shut the door first, please."

Oh, this is not good. Not good at all. She closes the door and sits opposite Gates, trying, with only moderate success, to look neutral.

"Detective, I'm sure that you've noticed that Serena Kaye is here."

"Yes, sir, yes I have. Hard to miss her." Oh, God, she shouldn't have said that. There's a brief silence followed by a slight upturn of Gates's lip.

"True."

If Becket were a betting woman—and she is, though she usually likes to pretend that she's above such things—she'd swear Gates is stifling a laugh. Might as well go for it. Maybe she has an ally here. "It's that dress, sir," Beckett says, briefly tilting her head in Serena's direction.

"Yeeessss," Gates whispers conspiratorially, before clearing her throat. "What you probably don't know, since the story has been kept quiet, is that Ms. Kaye was successful in, uh, recovering an extremely unusual and priceless Stradivarius that was stolen from the Cosmopolitan Art Museum's collection of musical instruments. The thief gained access through a vent under the eaves."

"I didn't know that she had, sir. So she cracked The Fiddler on the Roof case?"

"The very one, Detective. Ms. Kaye's recovery method was, uh—"

"Unorthodox?"

"Yes, that's a tactful if not entirely accurate way of putting it. At any rate, the violin is safe and sound."

"That's good news, sir, but I'm not sure what it has to do with me."

"The violin is here."

"Here, here? At the Twelfth?"

"Bingo, Detective."

"Because, sir?"

"Because you and Mr. Castle will be escorting Ms. Kaye and the Stradivarius to the museum, where she will be given a special commendation from the board. Champagne, caviar, the whole deal."

"You can't be serious, Captain. I mean, excuse me, but can't some body guards go with her, or, or patrolmen? Someone from Robbery? I'm a homicide detective. And I didn't, we didn't, you know, have anything to do with that case."

"I'm well aware of that," Gates says, looking over the top of her half glasses.

"May I ask if there's a reason that I have to go, I mean that I specifically, and Castle, have been given this assignment, sir?" She's more desperate by the second, having hideous visions of Serena lifting her glass of champagne, wrapping her arm around Castle's and taking a sip, licking her crimson-gash lips as she gazes into his eyes.

Gates clasps her hands and rests her elbows on her desk. "The museum is grateful not only to Ms. Kaye but to you and Mr. Castle. The three of you worked together on that case last year, solving both the murder of their executive director and the theft of a staggeringly expensive sculpture. They felt that any kind of celebratory gesture at the time, given the death of their director, would have been unseemly. The museum would like to commend all of you now, not just Ms. Kaye. This is not the sort of thing that goes unnoticed in certain circles. One PP feels that this would be a good opportunity to have the department center stage at an important civic moment. Hands across the cultural sea. I quote."

"Serena Kaye is a thief, sir. I don't care how noble her intent. And she, she—she profits from it."

"Detective."

"Yes, sir."

"I'm afraid that's an order."

"Yes, sir."

Gates raises her head to look through the window of her office. "I must say that she and Mr. Castle still seem to be getting along famously."

Beckett wonders if her superior can her teeth grinding. "Yes, sir, they do." She stands up and says, "Thank you, Captain. What time are we supposed to be at the museum?"

"Two thirty. You will have an escort, one car behind, one in front. The sergeant will have the violin ready for you when you go downstairs. He's expecting you at one forty-five."

"Will do, sir. At least I won't have any trouble locating Castle, since he's more or less glued to Serena's side." She has a thought. "Would you mind if I ran home for a moment?" She gestures to her jeans. "I'm really not dressed appropriately for a champagne reception."

Gates gives her a long, inscrutable look. "Not at all, Detective. I'll expect you back in an hour. That should give you enough time, shouldn't it? It will bring you here just around the time you need to leave for the museum."

"Right, sir."

She grabs her bag from her desk drawer and tells Ryan and Esposito she'll be back in an hour. "Keep an eye on Castle and the, uh, hood ornament, guys," she says before she stalks to the elevator. On her ride home she mentally goes through her wardrobe. She has to fight fire with fire, but she's not going to wear anything like that dress. Doesn't own anything like it, anyway. Besides, she's representing the NYPD, and can't go the Cosmofreakingpolitan Museum looking like someone undercover in Vice, much as she'd like to.

At home now, standing in her closet, she's choosing and rejecting item after item. Finally she takes a deep purple silk blouse with a plunging-just-enough neckline and a pencil skirt with a just-short-of-wanton slit at the back. She steps into her very best fuck-me shoes with four-inch heels, touches up her make up, and shakes her hair loose. That should do it, she says, eyeing herself in the bathroom mirror. Bring it on, Serena So Not O'Kaye.

Arriving at the precinct lobby with a minute to spare, she has just enough time to pose artfully by the door before the elevator disgorges Serena and Castle. She notes two simultaneous and opposite orbital reactions from the pair when they spot her. While Serena's eyes narrow appreciably, Castle's widen appreciatively. Good. Just what she was hoping for.

"Ready?" Beckett asks casually. Without waiting for a reply, she moves toward the desk sergeant and takes from him the trillion-dollar violin, which is nestled in a case. "Lookin' good, Beckett," he says quietly, giving her a wink.

When they reach the car, Beckett says, "Why don't you sit in the front, Castle? Serena and I will take the back. With the… fiddle… in between us." He chokes out some response, she's not quite sure what, and they all get in and buckle up. "Excuse me," Beckett says to Serena as she extracts a file folder from her bag. "There's some work that needs my attention right now." She flips it open, not quite wide enough for Serena to see inside, and as she reads makes the occasional small noise. A couple of times she marks a passage with a pen and clucks. The work that requires her attention is, in fact, a short story by Alice Munro that she downloaded and printed it before she left home. It's a great one, too. She looks out the window occasionally to chart their progress and when she sees that they're just two blocks away she closes the folder and returns it to her bag. "Here we are," she says. "Scene of the crime." And opens her door.

She barely manages to survive the reception, declining the Dom Perignon ("Sorry, I'm on duty") that Serena and Castle and the impeccably dressed museum-board crowd are obviously enjoying. She smiles at the right times, shakes the right hands, makes the right comments, and keeps an accurate count of how many times Serena has felt it necessary to put her hand on Castle's arm. Twelve. Jesus.

Finally, finally, she gets a moment more or less alone with Castle. "Do you have to play the part quite so well?" she asks.

"What?"

"Oh, come on Castle, that woman is all over you."

"I'm being polite, Beckett. Besides, I'm supposed to look single."

"Four-star review for you then, Castle. You two look like you're headed for a quickie."

End of conversation, such as it was, as Serena slithers back to them.

Back at the precinct, Beckett appears to head for the ladies room but instead makes for the back stairs. She has been struck by lightning. Her brain is fizzing, and it's not champagne. Since she hadn't had any. She walks down a flight to a corner that's always quiet and takes out her phone, but before placing a call stops to reflect. What she's doing, is she nuts? No. Absolutley not. Is it too soon? Hell, no, she thinks.

Kate Beckett is many, many things. But impetuous? Castle has the market on that. It could be his middle name, ahead of Edgar and Alexander. But right now, at four o'clock on a cool October afternoon, she is doing something precipitous. She punches in a number that her fiancé has on Favorites, but that she has had to look up. There's an answer after the second ring.

"Hello, is he in please? This is Detective Kate Beckett, NYPD." Will that be enough for her to get through? "Yes, of course, I'll hold."

She swipes a palm over her slightly damp forehead. "Good afternoon, Mayor Weldon. It's Kate Beckett."

TBC

A/N From a prompt by mobazan27: Serena Kaye reappears and thinks Castle is available.