A chamber thriller with a confined setting, “Burn” features a small ensemble in a tale of a gas station robbery that goes terribly wrong. Many viewers will think the movie does, too.

Your reaction to the picture will depend largely on how credible a character you find Melinda (Tilda Cobham-Harvey), a clerk at the misnamed Paradise Pumps, a 24-hour-a-day establishment in some unnamed town. She’s awkward but determined at social interaction, being introduced confronting, ever so civilly, a man who insists on smoking while filling his tank. As a means of just feeling something, she occasionally dips a finger into scalding coffee. And she is clearly smitten with local cop Officer Liu (Harry Shum, Jr.), a robotic by-the-books type who seems oblivious to her interest. On the night in question, she’s paired with Sheila (Suki Waterhouse), a brazenly cynical, sharp-tongued tart who enjoys baiting and berating her.

The pair are visited by a few oddball customers in the earlier period of their night shift—like a guy who saunters in to offer Sheila help with her tired feet; she understandably tells him to get lost. By their routine is abruptly broken when Billy (Josh Hutcherson) shows up. Though well-spoken and courteous enough at first, he abruptly pulls a gun and demands money from the cash register and the safe. The obviously unstable Melinda is drawn to the outlaw, who explains that he needs the cash to pay a debt he owes to a nasty biker gang, because the excitement he represents feeds into her fantasies. Surly Sheila, on the other hand, berates the guy for acting like a wuss—a ploy that ends badly.

Billy is understandably nonplussed at Melinda’s attention; he tries to decline as reasonably as he can when she urges him to take her along with him, apparently hoping for a weird Bonnie-and-Clyde sort of coupling. Eventually, as often happens in such stories, the tables are turned, and she is able to take advantage of him in a fashion that proves how sexually tormented she’s been.

But their time alone is constantly being interrupted—by other customers, by those bikers, by Liu, and by Sheila’s boyfriend Perry (Shiloh Fernandez), who drops by to pick her up and is irritated by the fact that she’s left. Melinda goes into contortions thinking up excuses to convince the various interlopers to leave (she infuriates Perry, for instance, when she finally lies about Sheila going off with a guy for a stint in a motel). And the relationship she’s desperately trying to build with Billy definitely goes south.

For a while writer-director Mike Gan, in his debut feature, is reasonably successful in screwing up the tension, and Cobham-Harvey shows considerable skill in shaping a character who’s quietly but obviously off-kilter emotionally. But as the script introduces twist after twist to add more and more juice to the mix, it grows increasingly silly, and in the final act Gan’s staging becomes less assured. The big finale comes close to being a mess, with one damned thing after another piling up.

Still, one has to give credit to the cast for their commitment to Gan’s darkly deadpan vision, Waterhouse and Shum are basically single-note characters, but Hutcherson puts his blandness to good use as Billy, and Fernandez has an unusual last scene. The movie was obviously made on a limited budget, but the crew—production designer Eric Whitney and cinematographer Jon Keng, in particular—use the limited cash to good effect.

“Burn” is decidedly flawed, but it should serve as a useful calling card for Gan, and a résumé enhancement for Cobham-Harvey.