At the Bastøy prison island for youths, the newest arrival, Erling "C19", soon becomes friends with Olav ("C1"). Under the rule of warden Håkon, Erling comes to terms with the harsh winter, the mistreatment by the staff, and the hated unit mandager Bråthen. Olav sees Bråthen molesting "C5", Ivar, a timid, shy boy in the laundry room. When he raises this with the warden, the boys are severely punished and C5 is reassigned to duties far away from Bråthen. However, after hearing that Bråthen expects him to be back in the laundry room later, Ivar drowns himself in the freezing waters on the beach.
Bråthen is apparently fired, to the overwhelming delight of the other boys. Olav is paroled and is planning to leave, but on the way to the departing boat, walks past Bråthen, who had been sent away on a shopping trip until Olav, the only witness to Ivar's rape, had left the island. Olav and Erling attack Bråthen in a fit of rage and they are locked in freezing solitary confinement as punishment. They are freed by Bjarne, one of the few Bastøy boys who has remained on the island as a caretaker.
Olav again pursues Bråthen, and the attack inspires a mass uprising. The boys ransack the prison and drive the staff away. Bråthen is repeatedly hanged in a barn, released and then beaten severely. The barn is in is set ablaze, but Erling drags him to safety. A navy ship is brought in and violently puts down the uprising. Olav and Erling manage to escape across a frozen fjord back to the mainland, but Erling falls through a gap in the ice close to shore and quickly freezes to death. Olav is shown later, older, as a member of a ship's crew, watching as they pass Bastøy island.