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.

When an agent investigating radioactivity levels in shellfish is killed in a beach hut fire, Steed and Cathy get involved in the heady worlds of nightclubbing, chess and lobster fishermen.

Tara's uncle is being held against his will in a country hotel. So Tara books in, and finds herself equally discouraged from leaving. Could the place also be home to the agent that Mother is missing?