For those unfamiliar with Jeanne DuPrau's young-adult books, Ember is an underground city built to preserve humanity after some unnamed cataclysm on the surface. But that was more than 200 years ago, and the clues left by The Builders to return to the surface have been lost. The people of Ember spend their lives maintaining the great generator that powers their city, and patching and recycling every possession.

But as the generator starts breaking down, and blackouts become a daily peril, the future of Ember falls to two teens: plucky Lina Mayfleet (Saoirse Ronan), who finds the ancient map left by the Builders, and brooding Doon Harlow (Harry Treadaway), who is determined to find and fix the generator - aided by gadgets cobbled together by his inventor father (Tim Robbins) and knowledge from the ancient worker Sul (Martin Landau).

But as Lina and Doon dig deeper, they discover an unpleasant truth about Ember and its seemingly avuncular mayor (Bill Murray).

Ember itself is a mechanical marvel, as director Gil Kenan (whose previous film was the computer-animated "Monster House") and his crew create a futuristic Art Deco look covered with two centuries of dust, decay and duct-tape patchwork. The sublime screenwriter Caroline Thompson ("Edward Scissorhands," "The Nightmare Before Christmas") adapts DuPrau's book into a sharp, engaging tale of teens taking charge - while mixing in a lightly subversive hint of political conspiracy.

"City of Ember" also proves that Ronan's Oscar nomination for "Atonement" was no fluke. Ronan gives little Lina a gawky charm, running through Ember's streets in her red messenger's cape and quietly, single-mindedly, saving the day. In a city going dark, Ronan's smile is a beacon of hope.

Sean P. Means can be reached at movies@sltrib.com or 801-257-8602. Send comments to livingeditor@ sltrib.com.