Given the shrill, overbaked, hyperbolic language that passes for debate between the Right and the Left these days, it's no wonder that the best place to address politics in film may be the horror genre.

Enter writer-director James DeMonaco's The Purge, a genuinely tense, effective home-invasion thriller starring Ethan Hawke (The Woman in the Fifth) and Lena Headey (Game of Thrones) that opens Friday.

A genre picture on its grisly, gritty outside, The Purge tries, not always successfully, to mount a critique of America's culture of violence and our growing obsession with wealth, which according to the film, has robbed us of our last shreds of compassion and empathy.

Heavy-handed? Perhaps, but the film is surprisingly effective.

The Purge is set in an alternate America that has solved its economic problems by instituting an annual ritual called the Purge: For 12 hours beginning at 7 p.m. every March 21, Americans are free to commit any crime. There will be no cops, ambulances, or fire engines on the streets, and no prosecution will follow once the dust settles.

The Purge saves America! It reduces unemployment and gets the economy going in a brutally efficient way: Once a year, the country is cleansed of all the poor, destitute, weak people who can't take care of themselves, all the parasites who live off the government's largesse.

It's Social Darwinism gone wild.

Hawke and Headey are very good as an upper-middle-class couple who made their money selling elite security systems - like the rest of the upper-crust, they can shut out the Purge with heavily fortified walls.

Things go horribly wrong when their son Charlie (Max Burkholder) gives in to empathy and lets a homeless drifter (Edwin Hodge) hide in the house. The film goes from tense to psychotic when a group of prep school kids lays siege to the house, demanding they be allowed to kill the drifter. Should the family protect the weak, or turn him out?

DeMonaco sneaks in the film's political message using RoboCop director Paul Verhoeven's tried-and-true method: The action is foregrounded against a constant stream of news reports, talk shows, and public-service ads, including "Purge Feeds," live footage of gangs of hunters killing and torturing fellow human beings. The dead are eulogized for their "sacrifice."

In its own clumsy, exaggerated way, The Purge takes a strong stand against our cultural obsession with violence and our tendency to conflate justice with vengeance.

The Purge **1/2 (out of four stars)

Directed by James DeMonaco.

With Ethan Hawke, Lena Headey,

Max Burkholder, Adelaide Kane.

Distributed by Universal Pictures.

Running time: 1 hour, 25 mins.

Parent's guide: R (profanity,

strong violence, torture).

Playing at: area theaters.