Maisie Hart's dad has a new job as live-in caretaker of a Victorian secondary school. Finding it hard to make friends, things change when Maisie tells her new classmates about something strange she saw the night before - a girl's face in the window of a gothic tower. The girls tell Maisie about the ghost of a Victorian pupil rumoured to wander the corridors. One particular student called Kat seems captivated by what Maisie has seen, and - both feeling like outsiders - they quickly become close. One night, though, Maisie comes across a tin box full of photos and notes. It's plainly decades old, and, squinting at the photos - belonged to a smiling girl who looks exactly like Kat...