Ella Longfield hears two young men flirting with teenage girls on the train and thinks nothing of it until she finds out that the two have just come out of prison. Her maternal instinct sounds the alarm, but she holds back. The next day, one of the girls, Anna Ballard, has disappeared. A year later, Ella is still plagued by feelings of guilt as she begins to receive threatening letters. An anniversary call reveals that Anna's friends and family have secrets. Anna's best friend Sarah doesn't seem to have told the whole truth that night either. It turns out that someone knows where Anna is - and has Ella in his sights.