A Scotland Yard detective is investigating a string of robberies and a murder, and the information he uncovers leads him to the estate of a wealthy but strange English family, who share their mansion with a group of nuns. The detective comes to suspect that neither the family nor the nuns is quite what they seem to be.

The sister of a famous, but as yet uncaught, criminal named "The Hexer" is murdered. Inspector Higgins of Scotland Yard believes that "The Hexer" will surface to take his revenge on his sister's killers, and plans to set a trap to finally capture him. However, soon bodies start piling up, and it looks as if "The Hexer" may get away yet again.

A man has been killed on the Dover/Boulogne car ferry. What is the connection between him and the attempts being made to kill Jane Kerrell, a young girl in her early twenties? As she speeds through the French countryside to the South of France, several attempts are made on her life as she is deliberately forced off the road by another car. But when she reports these attempts, the local Cap Ferrat Police Inspector and the sinister psychiatrist, Dr. Forla believe these attempts are in her imagination and Dr Forla, concludes that Jane is mentally disturbed. At her wits end Jane finds an ally in the young English painter, Paul Hedley who finally believes her life is in danger following an attempt to murder him. When Inspector Malling of Scotland Yard and Mr. Breese arrive in Cap Ferrat trying to uncover the connection between Jane and the murdered man on the ferry, this thrilling puzzle of international intrigue begins to unravel against the backdrop of the French Riviera.