A young woman's suicide at a London cinema box office is revealed to be murder, triggering a Scotland Yard investigation into a sinister criminal trio. Miss Darke, fearing for her life after confiding in Inspector French about a gambling scam and her friend's murder, becomes the next victim when her body is discovered. French's investigation uncovers a pattern of similar murders, all connected by the victims' employment and a mysterious stranger bearing a purple scar. The inquiry must navigate the shadowy world of illicit gambling and the desperate measures taken by criminals to conceal their operations, all while French races against time to prevent further deaths.

As the fifth novel featuring Inspector French, this work by Freeman Wills Crofts, a writer once celebrated as ‘The King of Detective Story Writers,’ is presented as a classic example of the Golden Age detective fiction. The premise sets up a complex investigation into a series of murders linked by a gambling syndicate and a distinctive perpetrator. The narrative is expected to focus on meticulous police work and logical deduction, characteristic of Crofts's style. The inclusion of a sinister stranger and a pattern of crimes suggests a tightly plotted mystery designed to engage readers with its intricate puzzle.