An agent has been killed at the secret dockyard where submarines are built. Steed investigates, and finds that a foreign spy has been arranging 'accidents' for him.

Illegal diamonds are swamping the market. The wife of a diamond merchant is murdered. Time for Steed and Gathy Gale to feign marriage.

Ear, nose and throat specialists are sneezing themselves to death, each after receiving a mysterious empty envelope. When Tara is kidnapped, Steed takes great care over his morning mail.