Melissa McCarthy is great at physical comedy, she's masterful at playing idiots, and she demonstrates a special brilliance on top of that: Nobody else in big-screen comedy is as gifted at turning a laughingstock into a tortured human being and back again in the span of 30 seconds. So I'm especially sad to report that Tammy, the first movie to feature McCarthy as a producer and a cowriter, is kind of a dud.

It's got a great McCarthy setup: A quintessential Crocs-wearing jackass with a dead-end fast-food job and a cheating husband watches her life fall apart. About 10 minutes into the movie, Tammy (played by McCarthy, who is 43) is defying the wishes of her mother (played by Allison Janney, age 54) and going on a cross-country road trip with her ailing grandmother (played by Susan Sarandon, age 67). Though it's jarring to see Sarandon playing a gray-haired elderly granny more than a decade too soon, she enjoys being the impish instigator of the film, egging her not-too-bright granddaughter into drunken, uncomfortable situations.

Tammy has a modest scope; the only real comedy set piece, an awkward robbery, has been thoroughly spoiled in the trailer. But a summer comedy with a modest scope is refreshing at this point: When it's just a bunch of funny women talking (Kathy Bates and Sandra Oh also star), Tammy's a lot of fun. But the rest of the time, it's not. All of the scenes with Mark Duplass, who plays possibly the least-convincing farmer in cinematic history, are cold, uncomfortable, and lacking in chemistry. Most of the blame has to fall on cowriter/director/McCarthy's husband Ben Falcone, who seems to think direction just involves standing the camera up in a nonintrusive place and hoping for the best. It's a shame to see such great actors playing such strong female characters stranded in a juddering, incompetent shrug of a movie.