Twenty years ago, teenager Callum Hinds went missing in England's Lake District. His uncle, suspected of having done the boy harm, was interviewed by the police. When the uncle committed suicide near his cottage in the Hanging Wood, everyone assumed it was a sign of guilt. The boy's body was never found.
Now Callum's sister, Orla Payne, who never believed in their uncle's guilt, has returned to the Lakes and taken up a job in a residential library close to the Hanging Wood. She wants to find the truth about Callum's disappearance. Prompted by historian Daniel Kind, she tries to interest DCI Hannah Scarlett, head of Cumbria's Cold Case Review Team. Hannah is reluctant, but when Orla dies in strange and shocking circumstances, Hannah determines to find the truth about what happened to Callum — and to Orla. Soon Hannah finds herself racing against time as the past casts long shadows on the sunlit landscape of the lakes.