Norwich is riddled with old chalk-mining tunnels, but no one's sure exactly how many. When Ruth is called in to investigate a set of human remains found in one of them, she notices the bones are almost translucent, a sign they were boiled soon after death. Once more, she finds herself at the helm of a murder investigation. Meanwhile, DCI Nelson is hunting for a missing homeless woman, Barbara, who he hears has gone "underground." Could she have disappeared into the labyrinth? And if so, is she connected to the body Ruth found? As Ruth, Nelson, and the rest of their team investigate the tunnels, they hear rumors of secret societies, cannibalism, and ritual killings. When a dead body is found with a map that appears to be of The Underground, they realize their quest to find the killer has only just begun -- and that there may be more bodies underfoot.