On Assignment Day, Lina draws the dreadful job of Pipeworks laborer—but Doon offers to trade places. That means Lina can be a messenger, which is what she'd wanted to do anyway.
During Lina's first day as a messenger, she delivers a short, mysterious message to the mayor from a guy named Looper.
Lina's home life deteriorates as Granny forgets to look out for toddler Poppy, so Lina asks their neighbor Mrs. Murdo to start looking in on them while Lina's at work.
When Lina delivers a message to her friend Clary at the greenhouse, they witness a guy coming back in from the Unknown Regions, going crazy with fear.
Clary agrees with Lina that there have to be some kind of answers outside of Ember.
Lina takes Poppy with her on a shopping trip. While picking out colored pencils, Lina loses track of Poppy—and then the power goes out for seven minutes, the longest it's ever been out. Lina is paralyzed with fear and guilt over losing her sister.
Luckily, Doon finds Poppy and keeps her til Lina can come grab her.
Granny is still obsessed with finding whatever her grandfather lost, and Lina comes home one day to find Poppy chewing on a message from a mysterious box. Lina rescues it.
She tries to get people to help her decipher the message, but they're not into it (like her friend Lizzie, who's pretty self-involved).
Doon agrees to look at the message, and once they figure out that it's referring to something in the Pipeworks, they go exploring, and find a secret locked room. They can't get in, but someone else obviously can.
Lina's grandmother gets sick and dies; the girls move in with Mrs. Murdo, who has been awesome to them this whole time. Lina's grateful for her help.
Lina tries to catch up with Lizzie, and finds schlepping a bunch of rare canned goods. It turns out that Looper stole them from the storeroom and is giving them to a few people he likes.
Talking with Doon, Lina puts together the puzzle pieces and realizes that Looper has been the one supplying the mayor with stolen goods. They report this to one of the guards.
Clary helps Lina figure out that the mysterious instructions are for how to exit Ember, so Lina and Doon go exploring in the Pipeworks and figure out how to work the boats that were left there.
Once the mayor puts a warrant out for the kids, Lina and Doon are forced to run. They decide to leave instructions on how to escape Ember. Lina's briefly captured by the mayor's guards, but escapes during a blackout.
She picks up Poppy from Mrs. Murdo, and meets up with Doon. They ride down the river and climb up to the earth's surface. Witnessing a sunrise for the first time blows Lina's mind.
They find a cavern that looks over Ember (which is underground), and Lina tosses a message down there, with instructions on how to leave Ember.