Twelve-year-old Petrel is an outcast living on an ancient "icebreaker" in a war-stricken world ruled by Anti-Machinists. The icebreaker has been following the same ocean course for 300 years. The ship's crew has forgotten its original purpose and has broken into three warring tribes. Everyone has a tribe except Petrel, whose parents were thrown overboard for alleged crimes. She has survived until now by living in the dark corners of the ship and speaking to no one except two large rats, Mister Smoke and Missus Slink. When a boy is discovered on an iceberg, the crew is immediately on alert. Petrel hides him on board, hoping he'll be her friend. What she doesn't know is that the ship guards a secret, held down deep in its belly, and the boy has been sent to seek and destroy it.