In 1960, on the remote Icelandic island of Flatey, the body of a Danish cryptographer is discovered. A young magistrate's assistant, Kjartan, is sent from Reykjavík for what should be a routine investigation. However, he quickly realizes this was no accident and finds himself entangled in a web of secrets connected to the legendary medieval manuscript, the Flatey Book. As Kjartan delves deeper, he must navigate the island's tight-lipped community and solve an ancient riddle to uncover a modern-day murderer hiding in plain sight among the isolated inhabitants.