Mitchell's series detective, Dame Beatrice Adela Lestrange Bradley, "psychiatrist and consulting psychologist to the Home Office with degrees from every university except Tokyo," and her assistant Laura Menzies have been invited to a house party organized around an elaborate Sherlock Holmes costume theme. The host is obnoxious and wealthy Sir Bohun Chantry, and the guest list is small but varied; a few friends, several family members, and Miss Menzies' fiancee, CID inspector Robert Gavin. With the possible exception of Bradley, Menzies and Gavin, everyone present has a reason to dislike and/or be indebted to Chantry.

Chosen for the part of Irene Adler is a young woman ostensibly employed as nursery governess to Chantry's youngest nephew. In the party's aftermath Chantry announces that they are engaged although no one can fathom why; the girl is a hard, grasping chippie, instinctively dishonest and a blackmailer, and if anything even more unpleasant than Chantry. It is no surprise when she turns up dead, stabbed through the heart and the weapon missing. The suspects include an unstable young man who was in love with (and spurned by) the dead woman, Chantry's out of wedlock son who stands to lose his inheritance if Chantry marries and produces legitimate offspring, and a pretty, sexually voracious houseguest eager to shed her husband for a richer partner.

Complicating Mrs Bradley's investigation are a a pair of thespians, a retired chorus girl, two small boys, and a Hound of the Baskervilles look-alike imprisoned in an abandoned railway station. She correctly interprets the clues and identifies the murderer -- but is there enough proof for Scotland Yard?