Heads up, all you tweens out there in Let's Go to the Moviesland. Today's word is "dystopia," as in the opposite of "utopia," 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 teenagers 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, ruling by tacit fear of the unknown ("Ours is the only light in a dark world") and promising to have a "task force" look into the generator problem. The religious just smile and sing their hymn, "This is all we know," as the lights dim.

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 fix 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 subterranean Dickensian tenement, is fascinating in its detail, that sort of retro-future tech you'd have to go back to "Brazil" to see done as well. Kids who have outgrown but seen "Bee Movie" and "Flushed Away" may connect to this depressing but complete world where your job-for-life is picked from a hat on "assignment day," and everything depends on very old, very elaborate plumbing.

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.

An ending that doesn't deliver a punch or a surprise notwithstanding, "City of Ember" is still good enough to turn a new generation of sci-fi fans on to the glories of movie dystopias, films that warn us of how things might turn out if we don't change our ways in their generation.