Robert Twyman a leading aeronautics magnate receives a note that his son, a piano player and contestant in a local variety show, would be killed if he does not confess. Next morning the son is found strangled by his piano in a locked house with only his father and butler present. Twyman is convinced a journalist and his son's rival piano player is the killer. Inspector Mallory arrests Twyman after Lady Felicia finds him standing over the dead body of the journalist by her piano. Father Brown notices both pianists had been somehow playing the same music and the answer is in a book given to him by Doctor Mordaunt Jackson; a psycho-analyst in Kembleford giving a lecture.