Desmond Merrion, late of Naval Intelligence, and his wife Mavis go to the little seaside town of Croylehaven to spend a holiday. The place is practically owned by the wealthy Croyle family, who live in the large, ugly Castle Croyle. The present head of the family, Lord Croyle, an old gentleman of eighty, generally known as Lucifer, is surrounded by a large number of more or less expectant heirs. It is peculiarly fitting, therefore, that Desmond Merrion, expert investigator, should be at hand when death enters the grim Victorian mansion and tragedy descends on the Croyle family.