Following the death of his fiancee Peggy at the hands of heroin smugglers, Dr David Keel vows to track down the killers and avenge his lost love. He is aided in his quest by a mysterious stranger, John Steed.

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.

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.

Dr. Keel resuscitates Elaine Bateman, the owner of a ballroom dancing school who has narrowly survived a murder attempt by gassing. When he returns to the school after dropping his scarf, he is arrested. His scarf has been used to strangle the woman.

When Keel attends a health conference in Geneva he is unaware that he is carrying, in microdot form, a vital new medical formula - unaware, that is, until he is arrested for murder.

Steed is trying to track down the organisers of a group who offer to spring any convict - even the dangerous ones - from prison if they're paid the right money. Keel impersonates a prisoner, and the trail seems to lead to a girl's finishing school.

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.

After an unsuccessful murder attempt against Sir Wilberforce Lungi, Steed asks Keel, an old friend of the pro-Western African leader, to investigate Lungi's secretary, Jacquetta Brown. Meanwhile, Steed flies to Africa to meet Lungi's tribalist rival, Shebro.

An agent has been killed at the secret dockyard where submarines are built. Steed investigates, and finds that a foreign spy has been arranging 'accidents' for him.

Keel, called to deal with a man seriously injured by an accident, quickly discovers that the man is suffering from gunshot wounds, and that he may be implicated in a recent diamond robbery.

Top-secret information is finding its way into the wrong hands, and the source seems to be a south coast fairground. Steed succeeds in getting hypnotised and winds up in the ghost-train tunnel. With frightening results, of course.

Dr Keel, treating the victims of a cyclone struck Mexican village, uncovers a number of food-poisonings; emergency cooking oil is, in fact, hydraulic fluid.

A visit to London to sign an oil treaty from a monarch whose life is under threat brings Steed and Keel into the world of dissidents and assassinations.

The discovery of the body of Schneider, a wanted Nazi war criminal, deep-frozen in a consignment of meat at London docks brings Steed and Keel into the shadowy world of Phoenix, a new and terrifying Fascist party in Britain.

An experimental vaccine is stolen, and the subsequent test proves deadly. Steed and Keel, suspecting that the human 'guinea pig' did not die directly from the vaccine, volunteer to be the next subjects.

An unusual and sprawling tale of heart disease, industrial unrest, and a consignment of rotten bananas.

How has a research centre developing radiation-proof material managed to irradiate one of its scientists? Once more, Steed submits himself to scientific testing.