Three little girls set off to school one sunny May morning. Within an hour, one of them is dead. Fifteen years later, Alison and Kitty are living separate lives. Kitty lives in a care home. She can't speak, and she has no memory of the accident that put her here, or her life before it. Art teacher Alison looks fine on the surface. But the surface is a lie. When a job in a prison comes up she decides to take it - this is her chance to finally make things right. But someone is watching Kitty and Alison. Someone who wants revenge for what happened that day. And only another life will do . . .