Heads up, all you 'tweens out there in Let's Go to the Movies land. Today's word is "dystopia," often used to describe works of science fiction that depict an Earth that has been polluted ("Blade Runner"), globally warmed ("Waterworld"), or Big Brother'd ("1984") into slavery.

" City of Ember" is a not-quite-classic addition to the genre, a dystopia that gets away with being derivative and somewhat prone to trite "thrill ride" action sequences. It could be the first dystopia many a 'tween or early teen moviegoer experiences, a movie about teens trying to save the human race.

"On the day the world ended," a narrator tells us, learned men in blue lab coats sent the last vestiges of the human race below ground to hold out for 200 years in the City of Ember. But those 200 years have passed. Knowledge has faded and the vast city is crumbling, degenerating as generations have grown up not really understanding how anything works, why they're underground or what was supposed to happen after 200 years.

The canned food supplies are running out. The generator that runs the lights is flickering. "Time," as the ticking clock we hear tells us, "is running out."

Whom do you turn to? The Mayor ( Bill Murray) is a corrupt empty suit. The religious just smile and sing.

But Lina (Saoirse Ronan of " Atonement") and Doon (Harry Treadaway of "Control") aren't giving up or accepting the official government line. She delves into the mystery of a missing box of instructions for the city. And Doon, the son of a tinkerer ( Tim Robbins) ignores his aged, "it's not my job" boss in the pipe works ( Martin Landau) and tries to get to the generator to have a crack at fixing it.

In a just universe, "City of Ember "would settle in as an instant cult film, the sci-fi movie you show kids to teach them science fiction. The set, a vast Dickensian tenement, is fascinating in its detail.

This dark and ambitiously simple adaptation of the Jeanne Duprau novel, by the great screenwriter Caroline Thompson ("Edward Scissorhands"), is both light entertainment and smart, subversive literature, urging the young to think for themselves, question authority and make their own destiny.

See the trailer and find local showtimes for "City of Ember."