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There are two kids in Ember.  That's a city underground.  Their names are Lina and Doon.  Ember is getting darker and it's old, so their electricity is burning out.  They try to find a way out.  Much earlier, the builders of Ember had left them a note.  It had been passed on through generations of mayors of Ember.  Lina's grandma is related to one of the mayors, so she has the box that contains the note.  But when Lina's grandma is looking through her stuff, she finds it (but doesn't know it) and Lina's little sister, Poppy, chews it up.  So Lina and Doon have to put the letters together.