Castle

Monday's explosive Castle puts Detective Kate Beckett in serious danger, but is her biggest threat a visiting agent from the FBI?

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When a serial killer begins leaving notes at crime scenes for Nikki Heat — the fictional detective Castle (Nathan Fillion) based on Beckett (Stana Katic) in his new book — the FBI's Jordan Shaw (guest star Dana Delany) shows up in New York.

"She comes along and basically sweeps that case out of Beckett's hands," Katic tells TVGuide.com. "That's kind of strange for Beckett to deal with. She's used to being the one in charge of any case."

Shaw and her team show up with loads of technology, and the shiny new toys catch Castle's eye. Though Shaw initially dismisses Castle, she soon learns his value. "At first she thinks he's just this puppy dog following Beckett around, but then she realizes he does have a brain too," Delany says.

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The episode serves as a reunion for Delany and Fillion, who played lovers in the short-lived Fox series Pasadena, and husband and wife on Desperate Housewives. The duo's chemistry translates to nice conflict: Impressed by Shaw's résumé and commanding presence in the precinct, Castle tells Beckett that Shaw is the "FBI you," causing a little bit of jealousy.

But Delany thinks Shaw represents far more to Beckett than a nemesis. "I think Jordan Shaw serves as kind of a ghost of Christmas future for Beckett," Delany says. "She sees you can be kick-ass in your job and also be married and have a child — you don't have to give up romance just to do your job."

Indeed, Castle and Beckett's back-and-forth romantic tension is turned up a notch when Castle spends the night at Beckett's apartment — to protect her, of course. But Delany thinks the sleepover may lead to something less innocent.

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"It's what people love about Castle," Delany says. "You have a great plot line in terms of the serial killer, but you also have the banter between Castle and Beckett. In the course of the drama, they become closer and more intimate. It advances their romance along."

The episode ends with a doozy of a cliff-hanger, which leaves Beckett's life hanging in the balance and promises more of Delany's character in next week's conclusion.

"Jordan Shaw is not done with the case yet," Delany says. "If people watch carefully, they will see clues to who the real serial killer is."

Part 1 of Castle's special TV event airs Monday at 10/9c on ABC.

Additional reporting by Natalie Abrams