When a roamer trades them an ancient book with only a few pages remaining, Lina and Doon return to Ember to seek the machine the book seems to describe in hopes that it will get their new community, Sparks, through the winter.
Ember, built as a last refuge for the human race, is running out of power two hundred years later, but twelve-year-old Lina thinks there may yet be hope when she finds part of an ancient message that may hold a key to getting out of the city.