On a jagged, bleak lava field just outside Reykjavik stands the Gallows Rock. Once a place of execution, it is now a tourist attraction. Until this morning, when a man was found hanging from it... The nail embedded in his chest proves it wasn't suicide. But when the police go to his flat, a further puzzle awaits: a four-year-old boy has been left there. He doesn't seem to have any link with the victim, his parents cannot be found, and his drawings show he witnessed something terrible. As detective Huldar hunts the killer, and child psychologist Freyja looks for the boy's parents, the mystery unfolds: a story of violence, entitlement, and revenge.