At Marseilles airport, Steed and Cathy find the courier they were due to meet dead. His sample of new liquid rocket fuel has been taken. Facing an array of enemy agents, the duo must recover the flask of Propellant 23.

Willis-Sopwoth, a top pharmaceutical firm, is being drained by cheap imitations of its products in foreign markets. The death of a member of staff while investigating the fraud brings Steed and Cathy face to face with a deadly artist and a plot to poison children.