"Whatever it is, I fear Greeks even when they bring gifts," says Laocoon in Virgil's "Aeneid" as he and his fellow Trojans contemplated the enormous wooden horse outside their fortified walls. His instincts are right: A contingent of Greek warriors lay huddled inside, waiting for the right moment to release the hatch and rout Troy.

"A History of Violence" is a Trojan horse with a similarly nasty surprise. Packed inside David Cronenberg's latest film, which presents itself as gift-wrapped, shoot-'em-up entertainment, is a sobering reflection on our culture's attitude toward violence. By making us root for a very likable, small-town hero (Viggo Mortensen), who must resort to extreme measures to protect himself and his family from thugs, the Canadian filmmaker is essentially forcing us to confront troubling questions. Is killing excused by moral imperative? Where does heroism end and vigilantism begin?

Cronenberg cannily references the Western mythology of Hollywood as he lures us into the heartland community of Tom Stall (Mortensen). Everyone knows each other in this sleepy Indiana hamlet, pop. 3,246, and nothing much happens except the raising of families, churchgoing and the pursuit of happiness.

Tom runs the most popular diner in Millbrook. He kisses his wife, Edie (Maria Bello), before he goes to work, cheerfully picks up trash on the street on his way into the restaurant. Edie's a devoted wife, a powerful lawyer and a good mother to their two children and, when the bedroom door is closed, she knows how to play naughty.

"Be quiet," she whispers into Tom's ear, skimped out in a cheerleader skirt. "My parents are in the next room."

The object of this seduction, however, isn't Tom. It's us. Cronenberg is hooking the audience into Tom's life and, by extension, small-town America. By the time two threatening strangers (Stephen McHattie and Greg Bryk) barge into Stall's Diner around closing time, we're ready to get behind Tom in anything he does. Little do we suspect our trust and empathy will become treacherous liabilities.

When Tom tells the intruders his kitchen is closed, the men become belligerent. Moments later, Tom, his staff and a few stragglers are staring at drawn guns. When one stranger aims his weapon at the waitress by the door, Tom swings his coffeepot into the other man's face, grabs his gun and pumps the second gunman with bullets. The first man plunges a knife into his foot before Tom dispatches him, too.

Tom is stunningly efficient, and we're too exhilarated to consider the gory aftermath. But Cronenberg refuses to spare us the spattery details. The faces of the slain -- in this and subsequent scenes -- aren't just bloodied; they're grotesquely disfigured.

Within 24 hours, Tom's a media hero. The assailants lie toe-tagged in the morgue. When Tom emerges, bandaged and triumphant from the hospital, Millbrook residents are waiting to applaud. Evil has been overcome -- or has it?

After his own personal Morning in America, Tom gets another unsavory visitor at the diner. This time, it's Carl Fogaty (Ed Harris), a grim individual who hides a terrible scar behind his shades. Flanked by dark-suited goons, he parks in front of Tom's counter and sarcastically compliments his heroism. He insists on calling him Joey.

"Who's Joey?" asks Tom, puzzled.

"You are," says Carl, who claims to know Tom from a previous chapter in his life. He wonders aloud how it is, exactly, that an unassuming diner proprietor could shoot so expertly. Although Tom gets the sheriff to chase Carl out of town, it's not the last Tom's family will hear of him. Despite Tom's insistence that he was "just lucky," Edie begins to wonder. Has Tom been living a double life? Who is the man that fathered her children, who sleeps by her side?

The filmmaker who gave us "Spider," "The Fly," "eXistenZ," "Crash" and "Naked Lunch" -- stories about shadow lives -- slowly begins to tear the gift-wrap. Tom's socially withdrawn son, Jack (Ashton Holmes), has been suffering at the hands of a high school bully. After he watches his father transform from coffee pourer to family defender, he changes, too.

"In this family, we do not solve problems by hitting people," says Tom, after Jack has been taken before the principal.

"No," retorts Jack. "In this family we shoot them."

And the audience erupts with laughter. But the gallows humor is really Cronenberg's subtle warning that our first reactions aren't necessarily the right ones. The possibility that Tom isn't everything we believed in is also taking uncomfortable root. Whether there's anything to this suspicion is the exciting business of the rest of the movie. As climactic events unfold, little touches earlier in the film -- such as Tom's daughter waking up screaming from a nightmare about monsters -- illustrate the ominous underpinnings that were always there.

"A History of Violence" forces us to confront our Pavlovian conditioning to violence, whether we are watching real military campaigns with living room detachment or whooping and hollering for fictional ones. It's not about popcorn heroism or the importance of protecting an increasingly troubled world against hostile invaders. It's just about why we're cheering.

A History of Violence (95 minutes, at Loews Georgetown) is rated R for extreme violence, graphic sexuality, profanity and some drug use.

Ed Harris, left, claims to have the goods on Viggo Mortensen in "A History of Violence."