On a sunny morning in June, Margaret Carpenter wakes up to find a new iPhone on her doorstep. She switches it on to find a text from her best friend, Charity Atwater. The problem is, Charity's been missing for over a month. Most people in town - even the police - think she's dead. Margaret and Charity have been lifelong friends. They share everything, know the most intimate details about one another . . . except for the destructive secret hidden from them both. A secret that will trigger a chain of events ending in tragedy, bloodshed, and death. And now Charity wants Margaret to know her story - the real story. In a narrative that takes place over one feverish day, Margaret follows a series of increasingly disquieting breadcrumbs as she forges deeper into the mystery of her best friend - a person she never truly knew at all.