Sir Bruno Lucer (Hugh Burden) is visited by his deputy Marcus Dodds (John Flint), just returned from international trips, who cryptically tells him the pigs walk backwards and the grass is black.
Cathy visits United Foods and Dressings Ltd, a major world supplier of fertilisers and grains, to investigate reported famine conditions in much of England, and is greeted by Eastow (Ian Shand), the PR officer, who offers her the assistance of the entire factory, but raises the ire of his director Appleton (Kenneth Keeling) who doesn't want her in his factory.
Sir Bruno's companion Octavia (Colette Wilde) arrives, and Bruno outlines his far more deadly plan for world domination - the synthetic recreation of the Black Plague, rather than simply creating famine. Bruno has scientists developing an inoculation for it, which will keep them safe and allow them to ransom the entire world.
Steed discovers that Sir Bruno, a known lover of Roman antiquities, is the silent chairman of UFD, and that sacks of contaminated grain bore a Roman monogram. He visits Sir Bruno under the pretext of selling Roman artefacts, but raises suspicion by first trying to sell a proto-Corinthian urn, and then giving the British Museum's telephone number as that of the seller. On his way out, he meets Dodds, who he recognises as a fascist political bully boy. Bruno orders him killed if he ever returns.
Marcus plots Bruno's death with Octavia, and when Lucius (Raymond Adamson) arrives from America for Bruno's conference and Bacchanal, he joins the plot. Meanwhile, Cathy offers to throw herself to the lions, and deliberately has herself found at night in one of the factory's labs by Sgt. Barnes (Colin Rix), who identifies himself over the 'phone as Cassio. The next morning, Barnes tells Appleton, Eastow and Steed that Cathy ordered a taxi to her hotel and left shortly before 9:00pm. A while later, Steed intercepts Barnes as he adds a white powder to one of the mixing machines, and forces information out of him.
Steed infiltrates the bacchanal, dressed in a toga and roman-style hair, but is spotted by Apollodorus, now revealed to be Eastow, and put in the cell where Cathy is trying to escape with the aid of an iron nail. Coming to collect Cathy to test the plague serum upon her, Lucius finds Steed missing, but he springs from a coffin and they fight sword against club.
Cathy is taken upstairs and presented with a goblet of poisoned wine, which she throws in Bruno's face as Steed appears, having despatched Lucius. The Avengers overcome the guards, but by the time they reach Bruno he has been stabbed and killed by Marcus.