Rizzoli needs a little help from Cagney.

On Rizzoli & Isles' 100th episode, airing Monday, Jane (Angie Harmon) goes undercover in a prison, where she'll meet an inmate played by Sharon Gless who leaves a lasting impression on her.

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"[Gless' character is] in for a lesser crime and she tries to befriend Jane, and she looks out for Jane," Harmon tells TVGuide.com. "Jane kind of brushes her off at first because she's in there with a job to do. ... But she does sort of change Jane's life."

By the end of the episode, which showrunner Jan Nash had previously told us serves as the "linchpin" of the final season, Jane is ready to act on her epiphany. But will she? And how far will she go?

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Angie Harmon, Rizzoli & IslesAngie Harmon, Rizzoli & Isles