Apocalypse how?

In "National Defense," a fearsomely funny new play by T.J. Edwards, this is the way the world ends: "Not in the hands of the superpowers, but in the hands of some dink terrorist with 12 pounds of plutonium."

Edwards' last play, "New York Mets," won this year's Helen Hayes Award as best new play, so "Defense," which opens the second summer repertory season of Woolly Mammoth, became something of a Washington Theater Event. And though some fine-tuning remains to be done, the disturbing "Defense" -- shot through with international paranoia, warning blasts of punk rock and spells of dire comedy and moonstruck romance -- fulfils that promise even as it delivers a threat.

As in "Mets," Edwards convenes a cluster of people on the fringe, but the denizens of "Defense" are as caustic as those of "Mets" were cuddly. In an all-but-abandoned schoolroom in an unnamed U.S. metropolis, this anonymous and argumentative band of zealots holds the key to the city's survival. The age of portable atomic weapons has arrived, and terrorism -- in the form of nuclear blackmail -- has come to America.

Our first encounter is with Marty, a disaffected punk who loiters in a long-unused room of the Catholic school and listens to his Clash tapes, a squatter with the blessing of near-deaf Sister Ann. As Marty pops out the window to the fire escape, the play begins.

It has slipped Sister Ann's dogma-befogged mind that Marty lives in the dusty classroom, so she admits Wally, a former student, into the room "to take pictures." Wally is now an FBI operative assigned to a surveillance job -- he sees himself as "sort of a cross between Hercule Poirot and Jungle Jim." When talkative Wally meets Mark, a terse, tough-nut CIA agent in charge of the operation, the small talk that fills up their "spy time" is like "Honeymooners" horseplay -- but with dark streaks.

Enmeshed in a private urban war, Mark and Wally are staking out a potential terrorist named "Bobby," who is due to receive a shipment of plutonium that could take out a large chunk of the unsuspecting city. The tedium of waiting is punctuated by colorful interruptions. Sister Ann bustles in and out with a string of funny nun-sequiturs. Mark's lover Rene arrives, and it's uncertain whether she's a double agent.

In fact, no one in this game of killing for causes is who he seems, and the exact nature of the events outside the schoolroom is purposefully clouded. The only constant is inconsistency -- Edwards keeps us off balance, unsure of whom to root for, what to believe. Like a life-or-death session of championship wrestling, the play develops as a painfully funny series of takedowns and reversals.

After a gun is pulled, the characters struggle for possession of the weapon, and each enjoys the sensation of being momentarily on top. But with neo-nihilist Marty's unexpected return, the situation suddenly slips out of anyone's control. "And that's the way the world ends," Mark says.

Edwards engineers a balance between low-key humor and tension, sentiment and morality, and handles his weighty issues without putting lectures in the mouths of his characters, who speak in gritty Mamet-dammit street talk. The playwright has a penchant for odd, telling detail: Mark and Wally play Hangman on the chalkboard ("I got a word that'll kill ya"), order pizza on a cellular telephone, swap crass cracks while the bomb ticks away.

Handsomely shaped by director Jeffrey B. Davis, the scenes are full of messy vigor, alternating between lulls and explosions, and the episodic structure steadily builds momentum. An important element is the smooth incorporation of jagged music. Marty's boom box erupts with the bristling sounds of early Clash, songs like "London Calling" and "Straight to Hell" illuminating his "no future" philosophy and providing a sound track for unfocused rage and endangered cities.

But Edwards loosens his grip near the end, and the tension slackens just where it's most needed. After the climactic confrontation that occurs outside the claustrophobic schoolroom, the characters' reactions are strangely abrupt and flat.

Edwards seems to have patterned Sister Ann after Christopher Durang's "Sister Mary Ignatius ...," and he trots her out primarily as an absurd comic device -- one of her unannounced entrances sparks a particularly neat bit of business with guns held behind the dim sister's back. But then Edwards gives her the deeply ironic last word, and unless Sister Ann, too, is something other than she seems, her benediction is hard to swallow.

The performances are sturdy and intuitive throughout. As Wally and Mark, Michael Willis and Jerry Whiddon have established an uneasy anti-rapport. Willis' Wally is a self-deprecating marshmallow with steel just beneath the surface; Whiddon's seen-it-all persona peels away to reveal a romantic core. Kyle Prue is an exposed nerve as Marty, a trusting child betrayed and enraged. Gra'inne Cassidy is quite capable of seeming all things to all three men as the enigmatic Rene. And despite her character's inconsistencies, Nancy Grosshans wins affection as the comically aggressive Sister Ann.

For the duration of the summer repertory, Woolly Mammoth is inhabiting a temporary second-floor space above the Washington Project for the Arts, and for "Defense" Lewis Folden has designed a convincingly forlorn schoolroom that eerily conjures memories of the Texas School Book Depository. The unusual stage area affords dramatic perspectives not available in conventional theaters, and suggests a world outside the window, as Edwards' play demands. National Defense, by T.J. Edwards. Directed by Jeffrey B. Davis; set and lighting, Lewis Folden. With Gra'inne Cassidy, Nancy Grosshans, Kyle Prue, Jerry Whiddon, Michael Willis. At Woolly Mammoth Theater (Washington Project for the Arts, Seventh and E streets NW), alternating with "Savage in Limbo" through Aug. 30.