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?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."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.