To stop his daughter seeing suspected conman Jeremy de Willoughby, Sir Thomas Waller has hired arch-criminal the Deacon to 'put the frighteners on'. Steed is on the trail of the Deacon and, with the aid of Dr. Keel, stops two thugs from seriously wounding de Willoughby. They set a trap for the Deacon and the lovers.
Frank Preston is released from prison and is intent on reclaiming the hidden proceeds of his robbery. So is Steed, who has been ordered to follow the man. So are a couple of thugs, who kidnap Carol. So is Preston's scheming wife, eager to get her hands on the one hundred thousand pounds hidden somewhere in the sewers.
Steed, attempting to break a gang of international diamond smugglers, is living in a bungalow near Heathrow Airport which was once owned by a suicide whom One-Ten suspects of involvement. Steed awakens one morning somewhat the worse for drink, when a phone call advises him to check the morning paper; the police are tracking a hit-and-run driver. Steed checks his car. It is damaged, and the front is covered with dried blood.
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.