Rizzoli & Isles' Angie Harmon on Jane's "Big Change" in the 100th Episode

Rizzoli & Isles is hitting triple digits with Angie Harmon pulling double duty.

The actress, who plays Jane Rizzoli on the drama, helmed Monday's 100th episode (9/8c, TNT), nearly five years after she was first supposed to step behind the camera.

"I was supposed to start directing in Season 2 and because of my schedule, it never worked out. I was just a little trepidatious because it was the 100th episode," Harmon tells TVGuide.com. "I found it to be very easy. The only thing I was worried about was being in the scenes and that was the easiest part. ... The cast and crew are fantastic and supportive. That was the only pressure that I felt -- it was the 100th episode and I wanted it to be special and I wanted it to look different."

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Not only is the show turning 100, but the milestone comes during its final season - there are five episodes left after this one - and serves as, according to showrunner Jan Nash, a "linchpin" for the remainder of the series. "The nature of the case challenges Jane in a way that sets up some of the stuff at the end of the season," she previously told TVGuide.com.

The hour finds Jane going undercover at a women's prison - tats and all - to suss out the murderer of two bikers. There, she meets an inmate played by Cagney herself, Sharon Gless. "It was so, so fun to act with her and such an honor," Harmon says. "I met her and she was such a staple in the strong law enforcement, powerful characters that women play on television now. She basically paved the way for the fact that I have a job and all of these women who are playing law enforcement officers. She started it."

Gless' character is not a suspect in the crime, but she is instrumental in steering Jane in a new direction once she closes the case. Nash had set out to challenge Jane's and Maura's (Sasha Alexander) "superpowers" this season, Maura's being her brain and Jane's being her prowess as a cop. The case does not land Jane in any life-or-death peril (again), but being a cop is all she's known in her adult life. The seeds for change were already planted last week, when Jane delivered a lecture at Quantico. With Maura recovering from surgery for her subdural hematoma and now writing a book, Jane "sees everyone around her changing and she's starting to embrace those changes as well, even though they're difficult, as they are for all of us," Harmon says.

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"I know what Jan really wanted to be translated was how everyone was helping somebody else," she continues. "That whole experience does change Jane's life. It sort of opens her mind and shows her that there are other opportunities and there are chances and these moments that come in all of our lives, and you either take them or watch them pass you by. It was fun watching her be courageous."

And it isn't just all talk on Jane's part. She does take a baby step by the end of the episode that will set up her ultimate ending.

"It starts in that episode. It seems small, but it's a big change. I think it's easy to make changes when you're miserable. But if you're happy and comfortable and 'fine' with your current situation, it's difficult to branch out or to look over and see what else is out there and change it," Harmon says. "I can't speak for other people, but I know I am pleased with where Jane goes and who she has become [by the finale]. I'm totally happy with it."

Rizzoli & Isles airs Mondays at 9/8c on TNT.