Is there an art-world equivalent of crying wolf? If so, Banksy has probably done it.

Banksy, the pseudonymous British street artist, has built his reputation on stunts  like inserting his own work among the masters’ in museums  that taunted the market in which his pieces sold for millions. But with his latest project, the documentary “Exit Through the Gift Shop,” he is laboring to convince audiences that he’s playing it straight.

The film, which opens on Friday in New York and California, follows Thierry Guetta, an amiable Frenchman who lives in Los Angeles and videotapes everything  or so we’re told. When Mr. Guetta and camera eventually tunnel into the world of street art  he was introduced to the scene through a cousin, the Parisian artist Space Invader  his enthusiastic recording melds nicely with the artists’ desire to have their otherwise ephemeral work documented. He captures that scene’s luminaries, like Shepard Fairey and Swoon working on rooftops and in alleys under cover of night.

It seems to be a natural fit for a documentary. But Mr. Guetta’s nonstop footage turns out to be unwatched (he has boxes and boxes of unlabeled tapes) and even when he cobbles something together after years of shooting, largely unwatchable. “He was maybe just somebody with mental problems who happened to have a camera,” Banksy says in the film.

So Banksy decides to take control of the material himself  or so we’re told. Robbed of his camera and prodded by Banksy, Mr. Guetta, meanwhile, morphs into a street artist, inventing an alter ego called Mr. Brainwash and staging an opening exhibition in Los Angeles that turns him into an overnight sensation, all of which is captured in “Exit Through the Gift Shop.”