Twelve-year-old Zac sets out on his first solo mission as a spy for the Government Investigation Bureau, piloting an amphibious submersible in hopes of preventing enemies from accessing a top-secret transmitter 4,000 feet below the ocean's surface.

While Gwin, a Fetcher, and her family defy the Devouts by sharing song and story, the Oyster's crew and the Sunkers continue fighting to bring light back to the dark world.

Teenaged Leyla McQueen, who lives beneath the ruins of a submerged London in 2099, joins a prestigious submersible race in the hope of clearing her father's name before encountering government corruption and a hidden world. Great Britain, the last days of the twenty-first century. Sea creatures swim among the ruins of Big Ben and the Tower of London, and citizens waver between fear and hope; fear of what lurks in the abyss, and hope that humanity...