When a child's bones are discovered near the site of an ancient Woodhenge, the police ask Dr Ruth Galloway, a forensic archaeologist at Norwich University, to help date them. The bones turn out to be two thousand years old, and not, as feared, those of a local girl who disappeared ten years ago. But then a second child goes missing, and DCI Harry Nelson receives a bizarre letter referring to ritual sacrifice, similar to a letter received after Lucy's disappearance. Now Ruth finds herself in danger from a killer who knows that she is helping the police.