What are the important events in city of ember?

At a rite of passage for all graduating students called Assignment Day, the mayor (Bill_Murray) stands before the graduating students as they choose, by lottery, what their occupations will be. Lina (Saoirse_Ronan) is assigned "Pipeworks Laborer" and Doon (Harry_Treadaway) is assigned "Messenger".

Lina goes home to find her grandmother searching for something in the closet, which turns out to be the lost metal box. Unable to piece the torn papers inside the box together, Lina nevertheless resolves to decipher their meaning and enlists Doon's help. As blackouts become more frequent, Lina and Doon realize that the information inside the box could lead to the salvation of Ember. Racing against time, the two follow the clues, cleverly maneuvering around corrupt politicians.

The pair realizes the document is a set of instructions on how to escape the city, and they search for the hidden exit. They also discover that the mayor has been hoarding canned food in a secret room. When they report the theft, they are declared traitors. Now on the run from the mayor's police, the pair puts their escape plan into action. They fetch Poppy, Lina's younger sister, and follow the instructions, which leads them to the surface where they see and feel the natural air and sky for the first time.

They then tie a message of their discovery to a rock and drop it back down to the city, where it is found by Doon's father (Tim_Robbins).