Lieutenant Colin Race of the Royal Navy, busy investigating the tragic deaths of his girlfriend and another agent, is walking along Wilbraham Crescent when Sheila Webb runs out of number 19 into his arms, distressed. A middle-aged man is found on the floor of the house, stabbed to death. Poirot travels to Dover to help Race to establish whether Sheila is a murderer. Sheila claims she did not know the victim, nor even Miss Pebmarsh, a blind woman who owns the house. Miss Pebmarsh was not at home when the body was found, and she denies having requested Sheila's services as a secretary by telephone earlier that day.
Poirot observes the complications of the case, including an unidentified murder victim, four clocks stopped at thirteen minutes past four in the room where the body was found, and a host of neighbours who say they saw nothing. Race is determined to find the suspected German mole within the Navy who caused his girlfriend's death, and in a state of grief he becomes increasingly close to Sheila. A wall of evidence begins to build up against Sheila, and then another victim is found brutally murdered.