As we posted earlier James Brolin will play the part of Castle's long lost father in upcoming episode/s. Castle creator Andrew Marlowe says "There will be a moment between father and son when Castle comes to a realization and the two men confront the realities of their relationship."
TV Guide also tells us about this super dramatic storyline:
The emotional episodes also showcase young Molly Quinn, whose character, Alexis, gets kidnapped. "She tries to escape any way she can but keeps getting thwarted," Quinn says. "Near the end, Alexis kind of falls apart."
Marlowe says Fillion lets the tears flow as he portrays the powerless dad who fears for his child's life. "We're giving Nathan the opportunity to show some colors and tackle a range of emotions he doesn't always get to show," says Marlowe, hinting that Castle's father figures into the kidnapping "in a very interesting way. He has a strong hand in how everything gets resolved."