In a ruined and toxic landscape, within a giant underground silo hundreds of stories deep, lives a society full of regulations they believe are meant to protect them. Then Sheriff Holston, who has unwaveringly upheld the silo's rules for years, breaks the greatest taboo of all: He asks to go outside. His decision unleashes a drastic series of events. His unlikely replacement - Juliette, a mechanic with no training in law, is about to learn how badly...

In the year 241, twelve-year-old Lina trades jobs on Assignment Day to be a Messenger to run to new places in her decaying but beloved city, perhaps even to glimpse Unknown Regions.