Nathan Fillion, Penny Johnson Jerald Nathan Fillion, Penny Johnson Jerald

In the Season 3 finale of ABC's Castle, Castle's boss died (R.I.P., Captain Montgomery!), he told his partner he loves her, and he almost put his own daughter in the line of fire. This guy is in need of some serious discipline! Enter Captain Victoria Gates (Penny Johnson Jerald), whose nickname, "Iron Gates," says it all. It seems that Castle (Nathan Fillion) is going to have a tough time being, well, Castle this season.

"She's not warm and cuddly," Seamus Dever, who plays Detective Kevin Ryan, says of new boss Gates. "So the days of us being protected, and the days of Castle being very welcome at the precinct are over."

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Captain Gates won't immediately cotton to the mystery writer-turned-homicide detective's unorthodox style.  In fact, his lack of formal police training may mean his days of tagging along with the team are numbered. "She's very much by the book; she doesn't take any shortcuts and she doesn't know Castle," Tamala Jones, who plays medical examiner Lanie Parish, says. "Castle had a relationship with the mayor and Captain Montgomery, and [Gates] doesn't have a relationship with the mayor, so some of his privileges might be getting cut. She's a lot harder, a lot tougher."

The one person who'll be happy about this turn of events is Castle's daughter Alexis (Molly Quinn). After her close call with a bullet (at a funeral no less), she'll start to realize just how much her dad is putting on the line for his side job. "Castle's not only risking his own life, he's risking Alexis' life, he's risking her grandmother's life. Alexis just wants him to realize that," Quinn says. "She thinks that by making him realize that, he'll back down. As she tries to get that across — what's even more scary — is that it's getting across, but he's like, 'I'm still going to do it. I still have to do this.'"

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Castle's determination to stay with the force will be met by resistance from Captain Gates, a woman who has no time for a man whose bulletproof vest says "WRITER" and whose far-fetched theories fly in the face of protocol. "Now we have a captain who doesn't care about Castle and who's not invested in Beckett. There's no trust between these people," Fillion says. "So you have a veritable stranger — who's not an easy woman — and you have Castle, who's very accustomed to dealing with women, but usually he finds himself being adored and charming. This is a woman who will not be charmed, so it's going to very difficult for Castle."

Season 4 of Castle premieres on Monday, Sept. 19 at 10/9c on ABC.