Any film with a title that sounds like a colonic evacuation really needs to be exceptional to justify its existence. “The Purge” isn’t. It’s just a routine home invasion tale with heavy-handed pretensions to social commentary.

Ethan Hawke plays James Sandin, a salesman for a home-security firm whose profits have boomed because of a new government policy designed to reduce the propensity for violence among the populace by channeling it into a single twelve-hour dusk-to-dawn period each year, a “purge” during which anybody can commit any crime with impunity. (There are only a few exceptions, like killing a government official, which is verboten.) This policy, initiated by those who are referred to in propaganda broadcasts as the “new founding fathers,” is endorsed, it would appear, by pliant behavioral scientists and privileged types who take the opportunity to haunt down and eradicate undesirables—the poor, the homeless, the misfits.

This is a premise that Rod Serling probably would have rejected for “The Twilight Zone” fifty years ago, but even granted its political and psychological implausibility, it might have served as the basis for some sharp satire. Instead what we get is a mostly plodding, curiously earnest commentary on the ills of the modern capitalist world. Sandin is a believer in the system, and at the appointed time puts his mansion’s formidable metal barricades in place so that he and his family—wife Mary (Lena Headey), teen daughter Zoey (Adelaide Kane) and precocious adolescent son Charlie (Max Burkholder)—can hunker down in supposed safety. But Zoey’s boyfriend Henry (Tony Oller), whom James is preventing from seeing the girl as often as he’d like, has secreted himself in the house with an agenda of his own, while Charlie offers sanctuary to a homeless man (Edwin Hodge) being pursued by a mob of bloodthirsty young marauders from the “best families” venting their vicious tendencies at the approved time. That leads the posse’s leader (a sneering Rhys Wakefield) to demand that the Sandins turn their quarry over or face an invasion themselves. And though James toys with the notion of giving in to the threat, that doesn’t work out. Soon he and his family are being stalked in their darkened house by a bevy of murderous purgers wearing Halloween masks and armed with hatchets, knives, and a wide array of firearms.

Writer-director James DeMonaco tries to extract tension from endless shots of James and Mary slithering along the hallways as the predators jump out at them from beyond the frame (or, in many instances, simply lurk menacingly in the background, waiting for the proper moment to strike). But we’ve seen this stuff far too many times to feel much beyond boredom, particularly when it’s as protracted as it is here. Naturally there’s one big confrontation when James has to face off against a passel of youngsters trying to kill him, and it provides the gore-hungry members of the audience with the sort of mayhem they’ve come to witness. But even that has a standard-issue feel, as does the frequently-employed cliché that has somebody step in to save any apparently doomed person just as he or she is about to get whacked.

The script does toss in a twist in the final twenty minutes when another group comes to the Sandins’ rescue, only to prove that some people don’t like to be overcharged for goods or to watch the Joneses do so well that they can’t keep up with them. But the attempt at social commentary—along with the final revelation of who the family’s ultimate savior turns out to be (an outcome that the least-seasoned filmgoer will predict without any trouble)—is delivered with a hysterical sledgehammer, rather like the one Brian Yuzna used in his little-seen but similarly themed “Society” back in 1989.

Hawke, who’s seen to much better advantage in the current “Before Midnight,” adopts an overly affected manner here, perhaps to distinguish his turn from the one he gave recently in “Sinister,” which also involved his interminably sneaking around darkened hallways, and Headey is simply dull. So are Kane and Burkholder as Zoey and Charlie (whose ostentatiously hideous little spying device would seem to be something that would play an important role in the denouement, but doesn’t). But by far the worst performances, however, come from Wakefield, who should have recognized that what he’s doing was already satirized by Michael Haneke in “Funny Games,” and by Arija Barekis as a suspiciously solicitous neighbor. Chris Mulkey, who once had prospects for a major career, can be briefly glimpsed as another resident of the subdivision. Technically the picture is no better than average, with Jacques Jouffret’s cinematography on the dank side, Melanie Jones’s production design uninspired and Nathan Whitehead’s score failing to generate much intensity.

One supposes we’re meant to feel satisfaction at how “The Purge” turns out. But mostly the satisfaction derives simply from the fact that it’s over.