The Great War is over. Europe is at peace, Germany’s scuttled fleet lies rusting in Scapa Flow, and Britannia once again rules the waves. Yet in the cold grey ocean north of Scotland, fishing boats and cargo steamers are vanishing, leaving no trace of wreckage or survivors. The Royal Navy dispatches HMS Pathfinder to investigate, suspecting treachery by a rival foreign power. Seconded to the mission are Lucas Avery and Sam Carter, two specialist agents from the Crown's newest and strangest department, who believe the disappearances may have a more unnatural cause. Amid the rocky archipelagos and isolated fishing villages of this distant sea, they soon find there are darker secrets at work than the Admiralty could have imagined – secrets the islanders will keep at any cost...