Steed has infiltrated a gang, led by Mason, which is implicated in Peggy's murder. Steed asks Keel to establish contact with a rival gang, and the doctor discovers that Vance's hitman, Spicer, has been sent to kill Steed.

Steed impersonates Riordan, a forger soon to be released from prison, and gains the trust of gang-leader Hooper. Things seem to be going well until Hooper's second-in-command, known as the Cardinal, announces that Riordan's girlfriend is outside waiting to see him.

Keel receives a 'phone call from one of his patients, and ends up assuming the identity of her missing husband, a scientist engaged in secret research. Shot in the chest, Keel needs a minor operation - but the anaesthetist has tampered with the oxygen.

A young girl, Carmelite Mendoza, has been kidnapped from a Caribbean island, but Steed suspects a political motive. His suspicions are confirmed when Vasco, the Mendoza family retainer, is observed killing the original kidnapper and abducting the girl himself.

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, 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.

Dr Keel is asked to help in the search for a man who has picked up a radioactive isotope, not knowing that it will quickly kill him and harm anyone else he comes into contact with. But Marko, fearing that the police want to find him because of his forged passport, has already gone into hiding.

Steed, investigating a number of probable arson cases, is led to the hairdressing salon of Olive and Jacques Beronne. He asks Keel to let Carol investigate the salon, but after she enters, there is an explosion from within

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.