This terrifically engaging debut feature by playwright Paul Downs Colaizzo is the best kind of “crowdpleaser”: one that earns every emotional beat that might seem formulaic in four out of five similar enterprises.

The tone of “Brittany,” and its emotional impact, reminds me of Amazon’s other heartfelt winner, “The Big Sick,” which netted Kumail Nanjiani and Emily V. Gordon an Oscar nod for original screenplay. Colaizzo should get one, too.

Colaizzo’s script weaves in enough detail to explain some of Brittany’s demons, but Bell sells the tough stuff too, doing more with a cautious look in the mirror and a slow smile than other performers can do with an Oscar-ready speech.

Colaizzo successfully walks a fine line between inspiration and caution, never presenting Brittany as a patronizing role model for weight loss, nor a clichéd case of inner beauty. The film grasps the complex nature of Brittany’s self-image without ignoring its dark side.

A fun ride, spiked with touching gravity, is not a shabby way to end the movie summer. Thanks to Jillian Bell, a comic force of nature with real dramatic chops, that’s what you get in Brittany Runs a Marathon.