Jillian Bell has been a consummate scene-stealer in just about every film in which she has appeared—among them, 2014's "22 Jump Street" and 2017's "Rough Night"—but with the emotionally stirring, beautifully crafted "Brittany Runs a Marathon," she graduates to full-fledged leading movie star. Front and center in every scene, Bell builds a protagonist in 28-year-old Brittany Forgler who will be achingly relatable to anyone who has ever struggled with body-image issues or felt as if they aren't in proper control of the life they want. Debuting writer-director Paul Downs Colaizzo has made a character study of genuine humor and pain, adversity and perseverance, his complicated title heroine worthy of love even when she herself does not yet believe it.

A Philadelphia native going through the motions of an entirely average life in Manhattan, Brittany whiles away her free time drinking and clubbing and sleeping into the afternoon. She originally moved to the Big Apple for a profession in the arts, but the closest she's gotten is a boring job as an Off-Off-Broadway cocktail server. When a doctor's check-up leads to some startling news—that her blood pressure is high, her liver isn't working as it should, and her weight and BMI place her in the 'morbidly obese' category—she makes a decision to begin working out. A wildly expensive gym membership is out of the question, so she starts running—a seemingly insurmountable achievement she takes one block at a time. Her new healthier lifestyle alienates her party-girl roommate Gretchen (Alice Lee), but she gains new running friends Seth (Micah Stock) and Catherine (Michaela Watkins). As the pounds burn off and a 5K race leads to a new goal of training for the NYC marathon, Brittany is smacked with a new reality check: her problems—personal hang-ups about love and trust and self-worth—run deeper than she thinks, and no amount of weight loss is going to fix them.

"Brittany Runs a Marathon" is destined to be called inspiring and moving, and it's certainly both of these things. What it avoids, however, is feeling prefabricated or manipulative, and this is key to how effective and affecting the film is. Brittany is piercingly authentic and sometimes challengingly flawed; a scene in which a slimmed-down but still insecure Brittany drinks too much and berates a heavier woman and her thinner beau at a family barbecue dares to even make her, for a few minutes, thoroughly unlikable. The growth that comes from the mistakes she makes only cements her messy, ultimately sympathetic, realness. Jillian Bell gives one of the year's most sharply, dynamically realized performances, her personal victories proving all the sweeter whenever she makes the necessary breakthroughs to becoming a fuller, happier person. Supporting turns are memorable across the board, with Utkarsh Ambudkar (2018's "Basmati Blues") an increasingly endearing foil to Brittany as Jern, a slacker with whom she finds herself co-housesitting, and Michaela Watkins (2019's "Good Boys") and Micah Stock (2018's "Life Itself") bringing their own sensitive layers to Brittany's new running pals Catherine and Seth.

"Brittany Runs a Marathon" is about so much more than running a marathon, her internal transformation altogether more profound than the outer one (over the course of filming, it deserves being mentioned, Jillian Bell lost forty pounds). Still, the title race will be on the viewers' minds, and when it arrives it is as gripping and uplifting as one could hope. Indeed, here is a picture, one of heart and wisdom and a certain acerbic acuity, with the power to potentially open eyes and drive audiences to strive toward their own goals. The tears, by the end, are cathartic in the best way, fully earned and nourishing.