Maisie finds it hard to make friends at her new Victorian secondary school - until she tells her new classmates about a girl's face in the window of the school's gothic tower. They tell Maisie about the ghost of a former pupil rumoured to wander the corridors. One student in particular seems captivated by what Maisie has seen, and - both feeling like outsiders - they quickly become close. Then one night Maisie comes across a tin box full of photos and notes, and she sees a smiling girl who looks exactly like her new friend. Now Maisie must help her new ghostly friend solve the mystery of what happened to her all those years before.