In the Soho section of London, several people are murdered by a phantom killer dressed in black and wearing gloves. When Inspector Hugh Patton of Scotland Yard reports on the case to his superior, Sir Philip, he meets the latter's fiancée, Clarinda Smith, a writer of mystery novels who announces that the identity of the phantom will be disclosed on the last page of her next book. Patton discovers that all of the murder victims were shipmates on a pleasure voyage, but his suspicions center on Joanna Filiati, the crippled proprietress of a Soho nightclub, and on Sir Philip, who appears to know more about the crimes than Patton has told him. After several more murders, Joanna and her physical therapist, Dr. Dalmar, attempt to leave the country, but Dr. Dalmar is killed at the airport. Joanna is saved by Patton, and the phantom is revealed to be Clarinda; all of her victims had been members of a vice racket and had raped her on the pleasure voyage. As Patton turns to the last page of the unfinished novel, Clarinda swallows a poison pill.
