For 300 years the massive icebreaker ship "Oyster" has been sailing around the world on the same course. Its residents, born and raised onboard, make up three warring tribes and have forgotten the ship's original purpose. Petrel, a girl without a tribe, lives alone in the ship's hidden places. When a mysterious boy stranded on an iceberg is brought aboard, Petrel hides him before the tribes can execute him. Although the boy has forgotten his own name, he knows he must destroy a terrible secret on the ship.