Mandy Patinkin and Keith Carradine, <EM>Criminal Minds</EM> Mandy Patinkin and Keith Carradine, Criminal Minds

In the pantheon of Criminal Minds madmen, Frank (Keith Carradine) deserves a special spot in hell. The twisted sexual sadist is said to have tortured and killed more than 100 people in the back of his RV over the course of 30 years. To add insult to fatality, he's the only suspect the FBI's Behavioral Analysis Unit ever let escape. "He is absolutely the worst. The most psychotic. The most compassionless," says exec producer Edward Allen Bernero. A sort of Hannibal Lecter on steroids. Or, as Mandy Patinkin, who plays BAU senior agent Gideon, puts it, "the Times Square of horror."

Chillingly, he's also based on a real person. While interviewing actual FBI profilers, Bernero and coexecutive producer Simon Mirren were told of a case involving one of the most prolific serial killers agents had ever come across. The culprit has yet to be caught.

When we last saw Frank, he led the team on a wild corpse chase into the Nevada desert, leaving his bizarre hostage and love interest, Jane (Amy Madigan), a gift of wind chimes made from the bones of his victims. Gideon let him get away in order to save a group of schoolchildren who would have otherwise died.

Now, in the creepy season finale (airing tonight at 9 pm/ET, on CBS), Frank is back, and he's taking his evil shop of horrors on the road in an episode that could be called "Mr. Psycho Goes to Washington." Gideon has something Frank wants, so the killer journeys to the FBI's neck of the woods to find it, slaying innocent people along the way — and even turning our hero into a suspect after Gideon's girlfriend is murdered.

"Without a doubt, this is a very personal case for Gideon," Patinkin says. "In terms of catching up with this man, it is something he would do until he took his last breath. He wants to annihilate this individual, and everything he stands for, from the face of this earth."

It won't be easy. Frank is as brilliant as he is heinous, and the finale explores his dark past and why he does the things he does. Once again, he's played to eerie perfection by Carradine (Deadwood), who says the role comes more easily to him than you might want to think. "With the criminally insane, it's an edge that any one of us could cross over, given the circumstances or a chemical imbalance," he says. "It's not difficult to imagine it, but it's also not a place where you want to spend that much time." After a long day inhabiting Frank, he says, "it's really nice to go home and say ‘hi' to the wife and the cats."

In what has to be the kookiest couple since Gomez and Morticia, Frank and Jane continue their romance — despite the fact that he once tried to mutilate her. The episode will attempt to explain why the two have a thing for each other, but Madigan has her own theory: "He's her Prince Charming," she says. "He completes her."

Other secrets will also be revealed. Throughout the episode, Hotch (Thomas Gibson) appears to be talking to a therapist. Is he getting all Tony Soprano on us, or is something else going on? Meanwhile, we learn that Gideon keeps a scrapbook of all the potential victims he's saved, which may lead to big trouble down the road. And we finally find out more about Agent Prentiss (Paget Brewster), whose family life was touched on briefly last month, with Kate Jackson playing her ambassador mother. Says Brewster, "There's always been some questions about how Prentiss' paperwork got past Hotch and Gideon and how she was accepted into the BAU. This episode will answer that question."

And what's the fate of Frank? Nobody's saying much. We do know the final confrontation takes place at D.C.'s Union Station (though it was filmed thousands of miles away at L.A.'s Union Station). Teases Bernero, "When we talked to Keith and Amy, we talked about a two- or three-episode arc with these characters, so we may or may not be ending that streak here." Frankly, that's terrifying.

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