Twelve-year-old Flavia de Luce is back at Buckshaw at last, but her homecoming is overshadowed by news of her father's illness. Forbidden from visiting Colonel de Luce in hospital, Flavia busies herself in the village, but she soon makes a macabre discover: the corpse of a reclusive woodcarver hanging upside down on the back of a door, in a house empty but for a curiously uncurious cat.

While the local constabulary are stumped, Flavia is soon piecing together a puzzle that connects a death by murderous gulls on a desolate island, a local woman said to be a witch, and a beloved children's author who terrified his own son - plus, of course, a certain tortoise-shell cat ...