John Hornby is the proud owner of a dazzling diamond business and when his nephews go into business with him, they are determined to be given their full share of responsibility. A consignment of diamonds is entrusted to the nephews who place their precious hoard in a safe overnight. But come the morning, the diamonds are missing and incredibly, the safe has been left untouched, all except for two blood smeared thumb prints and the inevitable presence of a mysterious Mr X. In one of Freeman's finest, will Dr Thorndyke, the erudite master of insight solve the enigma of the red thumb mark?