November, 1957. Six teenage girls walk in the Derbyshire mists, the first chills of winter in the air. They follow the old train tracks into the dark tunnel of the Cutting. Only five reappear on the other side. October, 2017. Feverishly fixated on a childhood friend, Mina's dying mother makes a plea: 'Find Valerie'. Following up on what seems like a routine death by natural causes, DC Connie Childs' old instincts kick in, pointing her back to one cold evening in 1957. But as Connie broadens her enquiries, the investigation begins to move increasingly close to home.