Thirteen-year-old Ava Bonney is unlike other children. Exceptionally bright, she has an obsessive interest in the rate at which dead animals decompose. The motorway she lives by regularly offers up roadkill, and in the dead of night Ava likes nothing better than to pull her latest discovery into her roadside den so she can study it. But one day when she arrives she stumbles across the body of fellow pupil, Mickey Grant. DI Seth Delahaye is given the case, one of the most challenging of his career. But Ava is not the sort of person who will step back and let someone else take charge when children like her are dying. She uses all her unusual skills and deep local knowledge to try to track down the serial killer in her community.