Every idealistic kid wants to think he can change the world at a stroke, and in the world of "The Manhattan Project," writer/director Marshall Brickman's teen thriller, he can.

The movie's hero, Paul Stephens (Christopher Collet), is a high school whiz kid who trains his genius on rarefied pranks -- painting a classmate's desk with an explosive chemical, or jimmying the lock on a glove compartment. That's enough to win him a pretty girlfriend (Cynthia Nixon), but he needs a larger canvas to paint on.

He finds it when John Mathewson (John Lithgow), a fancy physicist, invites him to his top-secret laboratory as a way to win the heart of Paul's divorced mother (Jill Eikenberry). Mathewson's lab manufactures a special, high-grade plutonium that the Army makes into bombs. So Paul gets the idea of breaking into the lab, stealing some of the plutonium and making a bomb of his own.

"The Manhattan Project" is magnificently photographed (by British cinematographer Billy Williams), and it sparkles with the kind of zingy, sharp-edged dialogue that hardly anyone writes anymore ("They'll lock you in a room somewhere and throw away the room," Mathewson says, trying to tell Paul the kind of trouble he's gotten himself into). And what's best about the dialogue is the way Brickman and cowriter Thomas Baum have given the kids the same brittle wit as the adults.

This is a movie about teen-agers that doesn't patronize them, which gives it a realistic, lived-in feel. The credit belongs equally to the performers. Nixon, a sweet-faced actress with blue saucer-eyes and straight blond hair, commands the screen with a relaxation and a clarity that's almost translucent. You see everything you need to know about her Jenny in the delightfully blunt, purposeful way she swoops in on Collet for a kiss.

Lithgow, always a remarkable character actor, builds a mathematical precision into his speech patterns that immediately places Mathewson in his milieu. He's nimbler, quicker, more animated than usual, shaping his gestures and expressions out of the rapid workings of a scientist's mind. And Collet, with his flat, laconic speech and wrinkle of a smile, captures the withdrawn arrogance of a young science jock -- it's an "I've got a secret" smugness, the secret being that he's smarter than everyone else.

But Collet has captured that attitude so completely that you never like him very much, which is part of what's wrong with "The Manhattan Project." The film endorses Paul as a hero, but it never tells you why you should endorse him, too -- certainly, it's not his personality. And it's not his idealism, either, since Brickman is careful to mix up and complicate his motives. Paul builds the bomb partly because he hates nuclear weapons, and partly to impress Jenny, to get back at his mother for having an affair with Mathewson and to simply show that he can do it.

This may make "The Manhattan Project" more complex, but it also makes it vague and, in a way, cowardly -- it would be nice to know exactly why Brickman approves of Paul. And if the mixed motives make Paul believable, hardly anything else in the movie is. While "The Manhattan Project" is meticulously researched, and each fact that the plot depends on is probably accurate, the way Brickman has put those facts together is flat-out implausible.

You could simply accept the implausibility, take it on faith and get on with the story, except that Brickman insists that you really believe it. So there are arduously long montages that show you, step by step, the theft and the creation of the bomb, as well as an extended sequence involving the return of the bomb to the authorities; and for some reason, Brickman has chosen to omit Philippe Sarde's orchestral score (which is inappropriate to a youth movie anyway) from most of these scenes so that they lie dead on the screen. Instead of the brio of a movie like "Thief," which got you inside the excitement of a complicated technical process, you get the cinematic equivalent of one of those interminably detailed New Yorker articles.

"The Manhattan Project" is only plausible, in fact, if you believe that the mere fact of youth solves everything. You're not supposed to admire Paul because he's adept, or charming, or wants to save the world, or has taken one giant step toward nuclear disarmament, or anything like that -- you're simply supposed to admire him because he's 16. That's easy enough for an audience of self-involved 16-year-olds, which is exactly the audience Brickman wants to capture. They've got the money, he's telling them what they want to hear -- and that, for anyone else, may be just a little too cynical to swallow.

The Manhattan Project, opening today at area theaters, is rated PG-13 and contains profanity.