Keel, reviving a young woman who has jumped into the Thames, is led to the Radeck State Circus, where the trapeze girl is guarded, her face covered by bandages because of an accident. The 'trapeze girl'proves to be the daughter of a defecting scientist, who will be used to force her father to return home. When Keel discovers this, he and Carol are captured by Zibbo the Clown, who will shoot Carol if the police investigation is not called off.

Steed trails a blackmailed civil servant to Brinkley House, a private zoo, only to observe the man throwing a package of money into the reptile pit. The package soon vanishes. The man is then ordered to steal a top secret file.

When an agent at the Ministry of Top-Secret Information is murdered, Steed and Tara have little difficulty in locating the man responsible. But when a handwriting test indicates that he is Boris Kartovski, someone Steed thought he had killed in Berlin 5 years before, our heroes find themselves drawn into the dangerous experiments of Dr. Constantine.

The directors of the Capital Land and Development Company are being murdered and the only clues Steed and Tara have are a massive footprint and a red nose.