Castle Castle

For Richard Castle, romance can be messy. Now he's about to face his daughter's love life.

On Monday's episode of Castle, at 10/9c on ABC, Castle (and viewers) will meet Alexis' first boyfriend. It turns out that the "cool dad" act is, well, an act.

"He's always telling his daughter to stretch her wings, but when she does, the reality of it is he's not prepared for it. He wants his little girl back and he's realizing more and more that she's growing up. It's very difficult for him," Nathan Fillion tells TVGuide.com. "He's not a cool dad, he's a protective dad."

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After years of Alexis (Molly Quinn

) watching over her father, new boyfriend Ashley (Secret Life of the American Teenager's Ken Baumman) will show her what she's been missing."He's showing her a new side of life that she hasn't really been able to get into because she's always had to take care of her dad," Quinn says.Letting his only daughter go will be complicated by Castle lacking a confidant for a reality check and advice. "He doesn't have any dads to turn to ask: What do I do?" Fillion says. "He has no idea what he's doing and his closest companion in this is his own daughter."

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This insecurity will lead to what Quinn calls a "hurricane reaction" when dad meets boyfriend. Luckily, Baumman has plenty of experience on the subject after three seasons on Secret Life. "Both are equally as terrified by girlfriends' fathers," he says of the similarity between Ashley and Secret Life's Ben.The meeting may be explosive at first, but Alexis' new relationship will have a more positive effect on her bond with her dad.

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"Their relationship is only growing stronger, and to get stronger, you have to discuss things," Quinn says. "That's what is really great about Alexis and her dad ... they never keep things bottled up. When they have a problem, they talk about it instantly."Check out this behind-the-scenes video with Fillion, Quinn and Baumman: