It is Charlotte's first night at boarding school. Before she goes to sleep, she sees from the window a corner of the new building. But when she wakes up, instead of the building there is a huge, dark cedar tree, and the girl in the next bed is not the girl who slept there last night. She calls Charlotte 'Clare', and says she is her younger sister Emily. Somehow, Charlotte has slipped back forty years to 1918 and the end of the First World War. Charlotte and Clare swap places ever night until one day Charlotte becomes trapped in 1918 and must find a way to return to her own time before the end of term.