Simon Baker and Robin Tunney, The Mentalist

Fans of The Mentalist are not going to want to miss the final moments of Thursday's episode.

"There's probably the biggest shock ending we've ever put up on the screen," creator Bruno Heller tells TVGuide.com "I think people will be intrigued and surprised by the whole episode."

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The episode finds Patrick Jane (Simon Baker, who also directed the episode) and the rest of the CBI investigating a triple homicide in small rural town. Further digging by Jane and Lisbon (Robin Tunney) reveals the killings are the work of serial cop killer. John Billingsley guest-stars as a self-professed psychic, whom Jane manipulates to find the real murderer.

But it wouldn't be November sweeps if Red John didn't have some part to play. "Red John does play into this episode in a surprising way," says Heller, who wrote the episode. Viewers will have to be paying attention, though, Tunney says. "It's just another small clue and Jane is the only one who knows," she says. "It leads him on his own personal investigation. The idea this year is that Simon's character was isolating himself more in the investigation, and this contributes to that."

The surprise ending sets up the introduction of guest star Pruitt Taylor Vince, who will first appear in the Dec. 9 episode as internal affairs investigator J.J. Laroche. The character is slated for several more episodes in the back half of the season.

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"The worry is that there's a mole in the CBI that's maybe working for Red John," Tunney says, noting that, at least initially, Lisbon and her team will still fully trust one another. "There's such a strong bond among them that nobody even suspects anything. In later episodes, people might have their [guard] up, but early on, everyone thinks the whole investigation is preposterous and that Laroche is on a witch hunt."

Laroche has reason to suspect foul play. After all, in Season 2, one of Red John's workers infiltrated the CBI and killed Agent Bosco (Terry Kinney). Like Bosco, Laroche and Jane won't exactly get along.

"Laroche immediately butts heads with Jane, and sparks will fly," Heller says. "Jane and Laroche are essentially looking for the same person, but for different reasons. It's a cat-and-mouse detective story."

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But will Laroche's "witch hunt" actually yield results? "Someone that we know very well and is very close to Jane is going to be revealed to be not who we always thought they were," Heller says. "Jane will use that to try and get closer to Red John. Very soon, things will be revealed that will start moving the Red John story forward in shocking and unexpected ways. Laroche will inadvertently reveal things that lead us toward that end."

The Mentalist airs Thursday at 10/9c on CBS.