Grace and her dad have just moved to a tiny, sleepy Devon village, desperate to put tragic events from their old life in London behind them. But their fresh start is derailed when the second-hand Echo in their new house comes alive at midnight, and tells Grace: I'm going to murder you. Grace works out it's an old recorded message, and the Echo has previously been used by four people in their village - all of whom are now dead. And Grace's passion for science and statistics comes alive. She knows within these accidental deaths, there must lie a murder - and she's going to work it out, using the methodology of a chemistry experiment. Observation; hypothesis; experimentation; results. No one believes her - except for her best friend, Maddy, who's desperate for Grace to face up to an agonising truth.