A girl employed in the box office of a London cinema falls into the power of a mysterious trio of crooks. A solicitor, learning of her predicament, sends her to the Yard. There she tells French the story of the Purple Sickle. That night she disappears, and next day her body is found floating in Southampton Water. Inquiries reveal the fact that other similar murders have taken place, and the further French goes into the affair the more girls he finds involved. Finally, after almost superhuman efforts, he learns the secret of the trio and runs them to earth.