The story takes place in the Ember, an underground city that is powered by a generator. The city is divided into sections, each with its own function such as the Pipeworks, the Blacksmith, the Power Plant, and the Administration Building. The citizens of Ember are governed by the Mayor and the Council of Elders.
The protagonist of the story is Lina Mayfleet, a young girl who lives in Ember with her grandmother. Lina is chosen to become a Messenger, a person who delivers messages throughout the city. She soon discovers a map that may lead to the outside world, and she teams up with her friend Doon Harrow to investigate the mystery.
The exposition also introduces other important characters, such as Lina's grandmother; Doon's father, who is the chief engineer of the Power Plant; and Clary, the head Messenger. The exposition sets up the main conflict of the story: Lina and Doon's quest to find the way out of Ember before the city's generator fails and the citizens are left in darkness.