Set in November 1918 on the opulent, castle-like island estate of the eccentric millionaire Claude Arkel, a young couple settle in as guests on Prosper Island. They find themselves quarantined in a shabby outpost far from Mr. Arkel's mansion - from which they learn three girls had gone missing. After she encounters a figure in the woods that may be the ghost of her long-lost friend Mary, Anna Filing Fort resolves to find out who Mr. Arkel really is, and what has become of the missing girls.