

Strange diseases are being reported in various parts of the world, and Steed's suspicion is that somebody has been tampering with the insecticides and fertilisers of United Foods and Dressings. Cathy Gale is captured whilst investigating, and will be the 'guinea pig' for a test on the new strain of bubonic plague.
Thanks to the clever briefing of the Lakin brothers a man charged with treason is found not guilty. Mrs Gale believes that Steed is implicated, and contacts the newspapers. Steed threatens her: either withdraw her allegations or pay the consequences. Soon Steed is on trial for Gale's murder.