The story centres on a girl starting at a boarding school, who finds one morning that she has travelled mysteriously back more than 40 years. In the past, the teachers and students call her "Clare". Charlotte and Clare change places each night, alternating between the years 1918 and Charlotte's time. Although Charlotte and Clare never meet, they communicate through diary notes in an exercise book. The story is wholly written from Charlotte's point of view: Clare never narrates. As it progresses, Charlotte becomes trapped in Clare's time, struggling to maintain her own identity.