It is 2250 in the land once known as Australia. Sixteen-year-old Luca will soon inherit the role of his father, Massa, and become the Deliverer. Once a year for generations, a Deliverer has descended below the earth’s surface to negotiate with terrifying, formerly human creatures known as Rats. In exchange for light sticks, the Rats promise to give the Toppers (those living above) another year of drinking water, since little is available on the surface.
The Toppers live under the rule of a prime minister and a council of nine leaders. Their henchmen, called Watchers or Amongus, monitor the people. In an effort to keep order, they have banned all art, books and emotion. Toppers wear devices that register any emotion, and Amongus punish or kill those who feel emotions for their infractions.
Massa has always been a mystery to Luca. He’s spoken little of Luca’s mother or of the things he experiences under the surface. He has always insisted Luca memorize the detailed set of directions to the underground water source, called the Aquifer, since Deliverers are the only ones privy to this path. When Massa vanishes after angering the leaders, Luca is unsure if his father is dead or alive. With the help a pirate named Seward, whom he discovers is his uncle, and a few other Toppers, Luca makes his descent below ground.
When he arrives, he realizes many of the things he and the Toppers have been taught are lies. The Rats are perfectly normal humans, not horribly-devolved creatures. They are good people, and they have no desire to keep water from the Toppers. Long ago, Topper leaders created these myths about the Rats to remain in control. Luca falls for a beautiful girl named Talya and learns that his own mother was a Rat. When his father tried to help her and Luca come to the surface many years earlier, his mother was killed by the leaders.
Talya and Seward return to the surface with Luca in order to locate Massa and right the wrongs of the political powers above. Meanwhile, the Topper world is in chaos. They think their Deliverer, Massa, has died and that his son was killed in a house fire. They fear there will be no way for them to get water, so they begin trying to blast through the earth’s surface with explosives to find the Aquifer themselves.
Luca and his companions find a weak but living Massa and set out to share the truth with their world. In the meantime, they get to know many of the Amongus soldiers and realize these Watchers have been deceived like everyone else. They become allies to Luca, and after much death and peril in his adventures, he is able to tell his people what he’s learned. He also believes an old book he left with the Rats will somehow offer them a new hope for their futures.