In the terrifically smart and genuinely inspiring comedy Brittany Runs a Marathon, Jillian Bell stars as Brittany, a 27-year-old New Yorker who decides to turn her life around. When we first meet Brittany, she has a dead-end job at a small theater and spends most of her nights out drinking and partying with friends. She gets in shape, takes up long-distance running and decides to give the New York City Marathon a try. The title more or less gives away the entire plot, which just goes to show that unpredictability can be overrated. Even a formulaic crowd-pleaser can be hard to resist if the formula is this well executed.

You know the movie will end with Brittany crossing the finish line, but the point is the journey, the long, agonizing buildup to the race. The writer-director Paul Downs Colaizzo drew the story from a close friend's experience, and he confronts issues of weight, body image and physical and mental health with a boldness that's rare in mainstream comedy.

One day Brittany visits a doctor hoping to procure some Adderall for recreational use. The doctor refuses and instead gives Brittany a diagnosis: Because she weighs 190 pounds and already has high blood pressure and an elevated heart rate, she's in danger of health risks down the road. Brittany, with typical good humor, needles the doctor for fat-shaming her, but he replies that he just wants her to be healthy and advises her to start eating better and exercising.

Brittany takes his words to heart, but it isn't easy to turn over a new leaf. She swings by her local gym, where she considers — and then quickly rejects — the idea of signing up for an expensive membership. Instead she starts running on her own, and though she's left panting after just one block, she decides to stick with it. Her cranky neighbor, Catherine (Michaela Watkins), extends an olive branch and invites her to join a running group. They befriend a third runner, Seth (Micah Stock), who's as much of a fitness novice as Brittany is.

Slowly but surely, everyone starts to improve. Brittany runs longer and longer distances and sheds several pounds in the process. She enjoys her boost in confidence and starts making new friends, lands a better-paying job and even rejoins the dating scene she'd given up on long ago. But as good as she looks and feels, she continues to struggle with feelings of worthlessness and despair, and she has a tendency to push her friends away, mistaking their compassion for pity.

You may recall the actress Jillian Bell from the Comedy Central series Workaholics, or perhaps her scene-stealing performance in the movie 22 Jump Street. Brittany is the richest, most complex role she's ever played, in part because her sense of humor is just one of her many emotional layers. We come to see how she uses comedy as a defense, encouraging others to laugh at her foibles so that she won't have to face them herself.

If this all makes Brittany Runs a Marathonsound like therapy, I suppose it is. But some therapy sessions are more entertaining than others. Nearly all the supporting characters are on hand to teach Brittany a life lesson or two, but they're all so fun to spend time with that it hardly matters.

Utkarsh Ambudkar, from The Mindy Project,plays a slacker artist type named Jern who gets thrown together with Brittany during an extended house-sitting gig. He's super-annoying and totally charming, and you know from their first friendly spat that they're made for each other. Alice Lee makes a nuanced villain as Brittany's self-absorbed roommate, Gretchen, an aspiring Instagram influencer who superficially cheers her on but seems to enjoy having, in Brittany's own words, "a fat sidekick."

And Watkins is especially good as Catherine, who seems so fit and successful and put together at first, until Brittany realizes she's going through intense emotional pain of her own. In less assured hands, Brittany Runs a Marathon might have been an advertisement for the benefits of weight loss or a simplistic ode to the joys of inner beauty. But instead, it's about how personal insecurity takes many forms, not all of which can be conquered, though maybe some of them can. And since we're all messed up in our own ways and just trying to do better, we might as well do it together, one mile at a time.

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