Charlotte McKinley asks Father Brown to exorcise her house, which she believes is haunted by the ghost of her sister Elspeth, who disappeared nine years earlier. The priest believes somebody is fabricating a ghost but then Charlotte disappears from a locked room. Inspector Valentine suspects Charlotte's husband Victor, a former doctor struck off for irresponsible prescription and previously believed to have killed Elspeth. A skeleton is unearthed but it is not Elspeth's and a coded message in a book leads Father Brown to discover the fates of both sisters along with the legend of the family's hidden treasure.