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?