Castle Castle

Castle's fourth season began with its main characters keeping secrets from each other — secrets that creator Andrew W. Marlowe called a "ticking bomb." On Monday's season finale (10/9c, ABC), that bomb will finally explode.

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Although Castle (Nathan Fillion

) has already learned Beckett's secret — that she remembers him saying "I love you" after she was shot — Beckett (Stana Katic) is not aware of this. But when the investigation into the murder of an Army vet puts Beckett on the trail of the man who shot her, Castle is forced to come clean about his lie: that he's secretly been investigating Beckett's mother's murder."He's been tasked with keeping [Beckett] away from that case for her own safety," Marlowe tells "And when that comes tumbling out, how is Beckett going to respond to the fact that Castle basically has lied to her and tried to control what she does?"The short answer to that question? Not well. "Both of them got knocked on their asses emotionally by some of the events of the finale," Marlowe teases. "It comes to the point where everything needs to come out in the open and they really have to deal with things head-on. We've done a lot of things in subtext, a lot of them trying to find their way and being very coy with each other. But the characters are at a point now where I think they're both ready to put their cards on the table."

Castle boss: Castle and Beckett's secrets are a "ticking bomb"

Marlowe is quick to note, however, that this verbal blowout is different from the heated exchange between Castle and Beckett in last season's finale. "The cards that they put on the table last year didn't really deal with how they felt about one another," he says. "[This year], everything comes out."Meanwhile, the re-emergence of Johanna Beckett's murder case also shines a light on the corrupt-cop conspiracy surrounding the case — in particular, the involvement of the late Capt. Roy Montgomery. How far will the team go to protect Montgomery's secret? And if they're caught, might heads roll?"It certainly puts everybody on edge," Marlowe says. "That shadow is far-reaching and it gets everybody wound up ... to the point where they don't know if they're responding appropriately or not. That Montgomery secret is hanging over the precinct. We know the kind of cop Capt. Gates [Penny Johnson Jerald] is, and how she would react if she found out these guys were pursuing that and not being honest."

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When the smoke clears, will Castle and Beckett be any closer to finally coming together, or will the new revelations tear them apart? Marlowe says either option has value. "Progress in their relationship has been stilted because of these emotional blocks that they have both put up and these secrets that they have to get to the other side of," he says. "I think [the finale] will deeply affect the relationship one way or the other, but either way, I think that allows for forward progress for us."And that progress may have you holding your breath all summer long. "We have not 100 percent landed the opening of next season," Marlowe says of this season's cliff-hanger. "But I will say, the way we're ending this season, people will want to know what happens in the immediate aftermath."Castle's Season 4 finale airs Monday at 10/9c on ABC.