England, 1938: The multimillionaire Charles Tremayne has disappeared. His trail leads to Ker, a small island on the edge of the Scilly Archipelago. Concerned, the psychoanalyst Lady Penelope Hatton, who is associated with Dr. Holmes, asks literature professor John Stableford for help, as Tremayne is her patient. Stableford agrees, but the investigation on-site proves difficult, as the ghost stories surrounding the island obscure a clear view of the facts. Nevertheless, it soon becomes clear to the occasional detective that Tremayne never left Ker alive. But was it an accident, suicide, or even murder? Stableford is puzzled until a rather inconspicuous clue helps him solve the mystery of Ker Island.