"Impulse," playing now at area theaters, is being advertised as though it is a sex fest, a peculiar emphasis for a movie that really aims to send chills rather than thrills down your spine. It is a story of a small town gone bad, as sacred American institutions -- the bank, the law, medicine -- turn weird and threatening with a vengeance.

For a thriller, "Impulse" is pretty frightening, until it veers out of control. It's as though screenwriters Bart Davis and Don Carlos Dunaway figured out everything but how to wind up the royal mess they created, and so settled for a final solution that is more phony than macabre. Former teen heartthrob Tim Matheson plays a nice young doctor who lives with Meg Tilly ("The Big Chill"), a wan young dancer who looks as though she has a bad complexion. They are summoned to Tilly's rural home town by the attempted suicide of her mother, and soon find that all is not well in America's heartland.

Four old men start playing kick the can in the middle of Main Street; a well-dressed burgher relieves himself in plain sight on the back of Tilly's car. A matron lifts some cash while waiting for the teller in the bank, a man picks a fight in a bar and some of the other customers are doing things more appropriate for Plato's Retreat than for a local pub. It gets worse.

The premise, as Matheson puts it as though a light bulb has just blinked on in his head, is that "people are acting out any urge that comes along -- minus the censoring factor." Sounds like an idea that would be good for at least a half hour of conversation at a dinner party, but it doesn't quite hold for 90 minutes of movie. The cause of the town's decline is reasonably obvious to a perspicacious viewer, and it won't be giving too much away to say that this could also be looked on as an ecological cautionary tale if the ending weren't so silly.

Hume Cronyn plays the town doctor (and nurse), a fatherly small-town general practitioner -- who comes down with an attack of weirdness. Tilly plays it annoyingly coy for most of the film, coming across as the type of woman who never seems to have anything to say. Matheson is more adept, a good-looking guy who practically reeks of reliability. The most interesting performance, aside from Cronyn's, is by Amy Stryker, as Tilly's friend Margo, whose flame into madness is plausible and frightening.

It's safe to say that director Graham Baker had few artistic pretensions. It is strictly commercial fare, and, at regular intervals, provokes clenched fists and shrieks with professional skill. It also, perhaps unwittingly, backs up every mother's claim that good manners are necessary for society to function: Once you break the social contract in small ways, the decline and fall of civilization can't be far behind.