Castle Asks the Unthinkable: Is Castle a Killer?

Monday's episode of Castle poses a challenging question for its detectives: Is mystery writer Rick Castle capable of the murders usually reserved for the pages of his fiction?

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When Beckett (Stana Katic) & Co. begin investigating a ritualistic killing, all the evidence points to Castle (Nathan Fillion) as the killer. Although Beckett wants to give her new lover the benefit of the doubt, the facts become increasingly harder to ignore. "I think that Beckett's particularly sensitive now that she's in this relationship with him," creator Andrew W. Marlowe tells TVGuide.com. "There's also the issue of whether Beckett has an objective point of view on what she's seeing. Of course it's tough for [the team] to believe, but they're met with more and more evidence. It makes them question how well they know this guy."

While we're fairly certain the star of the show will eventually be cleared, Beckett will uncover some other startling information about Castle that could do much more lasting damage. "What's revealed in the episode is that Castle may have been having an affair with [the victim] at the same time he was dating Beckett," Marlowe says. "Of course, that goes to the heart of all of Beckett's insecurities. I think there are all sorts of fears and concerns. It's a double nightmare that he may have been seeing somebody else at the same time that he was seeing her, that he may have been keeping secrets."

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For Marlowe, making this serious turn is an important move in a season that's mostly had lots of fun with Castle and Beckett trying to keep their relationship a secret. "I felt like the run this year needed to be anchored," Marlowe says. "We've had a great deal of fun before this episode and will have a great deal of fun after it. But I wanted to make sure that the gravity was there in the series."

And the fun will indeed return in a future episode when the team investigates the murder of a rock star whose band was being filmed as part of a documentary. "Our guys basically walk into somebody else's documentary, and it gives our characters an opportunity to both hide and mug for the camera," Marlowe says. "Beckett's kind of a private person who doesn't necessarily love having her space invaded. And Castle and Beckett have a secret they don't want the world knowing, so the fact that they're operating under these omnipresent cameras adds a little bit of strain for the two of them."

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But viewers also shouldn't expect the secret to stay hidden forever. Now that Martha (Susan Sullivan), Alexis (Molly Quinn) and Det. Ryan (Seamus Dever) know, Marlowe concedes that it's only a matter of time before everyone else finds out. "At a certain point we're going to move beyond it," he promises. "There's only so much storytelling we can sustain and have fun with it."

So, does all this talk of Castle and Beckett's relationship mean that they come out of Monday's episode unscathed? Not necessarily.

"[This season is] an ongoing process of challenging the relationship and seeing what it's made of," Marlowe says. "In every relationship, there are difficult moments, and it's how you choose to respond to those moments that define a relationship. Sometimes you respond in a way that things seem to be OK, but there are lingering effects that come up later. We certainly don't walk away from this episode with Castle and Beckett on the rocks... but we're not taking anything off the table. We walk away with the two of them in a different place."

Castle airs Mondays at 10/9c on ABC.