Nathan Fillion

During Castle's recent two-parter, Rick's former muse dropped a bombshell just before she was seemingly killed.

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Sophia (guest star Jennifer Beals) hinted to Castle (Nathan Fillion) that his father may have had ties to the CIA, which would explain both his absence from Castle's life as well as the ease with which Castle was able to shadow a federal agent. The question remains: Is Sophia — who was revealed to actually be a Russian sleeper agent — being honest?

"I think [Castle] is evaluating whether Sophia was telling him the truth," creator and executive producer Andrew W. Marlowe tells TVGuide.com. "He doesn't have any way to really access it. So he is trying to figure out how to follow up on it, but I am not sure that he knows how."

That said, Marlowe says viewers will eventually get to see Castle pursue the possibility. It just may not happen anytime soon. "We are planting seeds for further storytelling assuming that we get good news on Season 5," he says, noting that the story might surface once more this year. "I haven't decided yet, but it is something I absolutely wanted to bring up. He is going to pursue it, but it is not something that he is going to pursue immediately. I think that he is still wrestling with it."

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Sophia rocked the boat in other ways too. Her re-entry into Castle's life has no doubt forced Rick's current partner-in-crime-solving, Detective Beckett (Stana Katic), to reconsider where she stands with Castle. "Beckett is looking at Castle's relationship with Sophia as an indication of the kind of person Castle is," Marlowe says. "This is a relationship that may mirror hers. This was another person that he followed, a muse. Beckett felt special going into the two-parter, but she was a little challenged there. I think it caused Beckett to question whether she was somebody special in Castle's life and whether the 'I love you' meant something."

Of course, Castle still doesn't know that Beckett heard him say those three little words in the Season 3 finale. Could this run-in with Sophia inspire Beckett to come clean? "It does start to push her down the road," Marlowe teases.

Regardless of how insecure Beckett may feel, Marlowe says Castle's reunion with Sophia changed how he sees Beckett. "I think the audience recognizes that Beckett has a different place in Castle's life than maybe Sophia did," Marlowe says. "I think that is one of things that attracts him to Beckett is that she is the kind of woman who challenges him and intrigues him. The question is how different is she [from Sophia]? Can their relationship sustain the two of them getting together?"

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The duo may find out sooner than you think. In an April episode, titled "47 Seconds," Castle and Beckett have to piece together the clues to find out who is responsible for a bombing at an anti-Wall Street rally. The life-threatening case may inspire them to seize the day, Marlowe says.

"It does make Castle and Beckett start questioning what they're doing in life," Marlowe says. "[The victims] had plans... and then suddenly their lives were over. So it makes you think: 'Should I wait or should I act now?' Those thoughts are definitely on their minds."

Castle
airs Mondays at 10/9c on ABC.