Marvik is surprised to receive an unexpected summons to meet Helen Shannon, a woman he helped on his first mission. When a body is discovered in her flat, Marvik is convinced that Helen is being framed for murder, but why and by whom? Before he has a chance to follow it up, the head of the National Marine Intelligence Squad, Detective Chief Superintendent Crowder, asks Marvik to investigate the disappearance of a salvage vessel, the Mary Jo, which went missing in 2003. As Marvik delves into the past, it becomes clear that he faces a desperate battle to keep Helen and others safe from a ruthless assassin - one who will stop at nothing in order to protect the secret of the Mary Jo's last voyage from ever being exposed.