The Avengers are tasked to investigate the Lakin brothers, two elderly lawyers with an amazing success record, having recently had an obvious traitor acquitted. In order to find out the secret of their success, Steed takes advice from them on how to murder Mrs. Gale and is then defended by them following her 'death'.
A man whom Steed was to escort to New York disappears, before re-surfacing at the Adelphi Park Retirement Home. Then his neighbour turns up there. All the residents at the home are extremely wealthy and have named their next of kin as beneficiaries to avoid death duties. Mrs. Gale gets a job as assistant matron there to discover what is going on.
A young government official staying at a holiday camp is killed and replaced by an exact double, part of a plan to gain information. Two other doppelgangers are also part of the plot, being hatched in the camp, and Mrs. Gale is sent to deal with the false Steed. Hopefully, she will get the right one.
The Nutshell is the name of a secure underground bunker, to be used by the government in the event of a nuclear war and Steed and Mrs. Gale are shown around it. When military secrets start to be leaked the finger of suspicion points to Steed as a traitor and it is down to Mrs. Gale to clear her colleague's name.
Steed attends the funeral of his former batman Wrightson, who had become a draughtsman on a modest salary in civilian life. However, at the will reading everyone is shocked to learn that he was exceedingly rich and apparently by honest means. Steed and Mrs. Gale investigate and uncover an ambitious plot involving insider trading.
Michael Dyter, the newly elected M.P. for South East Anglia, is assassinated after he has threatened to expose a government plot concerning the theft of a nuclear warhead. Mrs. Gale replaces him as a potential political candidate but she and Steed discover that Dyter is actually alive and in possession of the war-head, which he intends to set off on November the Fifth.
J. P. Spagge is an international criminal mastermind and The Avengers have a plan to trap him. They pose as thieves out to steal three million pounds worth of gold from a holding vault, though the heist goes wrong when Mrs. Gale is caught. In the event,this turns out to be merely a trick on the part of Spagge, to determine whether or not the 'robbers' are genuine. The real robbery is yet to come.
Steed is involved in the escort of a pair of corneas to be used in a graft to give sight to Martin Halvarsson, a millionaire who has been blind since the Second World War. A doctor who opines that there is something wrong with the procedure is murdered and then it turns out that the donor of the corneas was also killed, apparently by one of Halvarsson's staff. The Avengers investigate.
Steed discovers that somebody is flooding the market with cheap imitations of medical products being manufactured by the Willis-Sopwith Pharmaceutical Company. Furthermore, there is a plan to distribute poisoned medicines in an oil-rich Middle Eastern country to start a revolution and drive out the British companies there.
When a spate of strange diseases breaks out on a global scale, the Avengers find out that grain being distributed by the United Food and Dressing Limited, has been deliberately tampered with and infected with ergot.Whilst investigating the firm's headquarters Mrs. Gale is caught and whisked off to a reconstruction of Ancient Rome, ruled by a mad, would-be Caesar called Bruno. His aim is to unleash bubonic plague upon the world and intends to test its effects on Mrs. Gale.
Leaving a Chinese restaurant Steed picks up the wrong coat and finds a cheque to the value of £5,000 drawn to a bank in Hong Kong. It becomes evident that the restaurant is the front for an international gold smuggling operation, run by big wig Mr. Lo. However, the Avengers are surprised to encounter Major Ruse, a modern Robin Hood, and his band of ex-servicemen who are exploiting the racket for a worthy cause.
Mrs. Gale is invited to spend a weekend at the stately home of Sir Cavalier Rasagne, an authority on medieval costume. However, when she arrives there is no sign of her host and before long it becomes clear that she has been set up by a man called Martin Goodman, who is planning a slow and sadistic revenge for the fact that, years earlier, he believes she broke his heart. Steed has to find her.
A Middle eastern emir, Abdulla Akaba, comes to London as he does every year for his health check up by specialist doctor Spender. Steed believes that the emir will be killed by poisoning and poses as a chef at the hotel where he is staying.Poisoned mushrooms are detected but the emir actually dies of what appear to be natural causes. Has Dr. Spender engineered this ? Or a member of the emir's staff? And why? The Avengers investigate.
All of the country's early warning systems bar one go off, mistakenly suggesting a nuclear attack. When land is about to be sold, adjacent to the one that was not sounded, interested buyers are invited to a fancy dress party on board a train. Steed, dressed as a gambler, and Mrs. Gale, a monk - and later a highwaywoman - join the party, where they meet the likes of Robin Hood and Napoleon Bonaparte and have to decide which one is a killer and why?
Snowy, an albino elephant, is stolen from Noah's Ark, a specialist zoo run by Noah Marshall, specializing in distributing rare animals. Mrs. Gale's experience of living in Africa enables her to investigate when she puts herself forward as an ex-big game hunter applying for a position at the zoo. She uncovers an illegal ivory trade but ends up in the big cat's cage. . .
The late reverend Harbottle was not what he seemed, as the gun and the antique German doll containing a micro-film,found in his belongings testify. In fact he was a member of Bibliotek, outwardly a religious organization but actually a front for a global crime syndicate. Posing as a man of the cloth, Steed enters their ranks and discovers a plot by certain factions to kill their leader, the bishop of Winnipeg. When the bishop perishes, Steed finds himself up against his deputy,gun-toting Sister Johnson.
When five British agents working on the Corinthia espionage line between Austria and Hungary disappear, Steed is sent to investigate. Sadly for him, the sole survivor, Hal Anderson, points the finger at him as the traitor, and he sent to a brutal interrogation centre where a man known as the Wringer uses torture as part of his methods. Mrs. Gale does not doubt Steed's integrity and uncovers gross, not to say, criminal, negligence on the part of her superiors.
Steed attends the funeral of a former colleague and not only suspects foul play but is struck by the fact that the tiny, isolated Cornish churchyard contains the corpses of several worthies hardly to be associated with the area. His investigations lead to a sinister assassination bureau, Mandrake Investments, who bury poison victims in the churchyard because its proximity to an old tin mine means that the nature of the soil prevents the detection of the poison.
An agent is killed at a wine shop and Steed realises that one of two brothers ,Frederick Paignton, who work in the shop has been blackmailed by his boss into the murder. In the shop's cellar there is a dark room, used to develop military secrets on microfilm. Frederick's brother Allan's fiancee Marion, meanwhile,, is being forced to shut down the security system at the research station where she works. Mrs. Gale investigates the latter whilst Steed employs his nose for a good vintage.
Whilst protecting the race horse of a visiting dignitary in England for the current racing season Steed observes that George Meadows, owner of the livery stables, is behaving oddly. This leads him to uncover a murder organization in which jockeys with high-powered binoculars single out chosen victims and kill them with poison darts.
Angered by a noisy motor bike gang who ride on their land, elderly sisters Cynthia and Ermyntrude Peck, who claim to be witches, cast a spell which causes all the motor-bikes to stall. Not only that but almost every electronic and mechanical object in the area also tends to malfunction. Mrs. Gale in her leathers joins the gang to ask permission from the sisters to ride on their property and in the process discovers that these two sweet old dears may actually be in possession of a sophisticated long range jamming device. They may have built a better mouse-trap but it ...
Five years earlier the British Secret Service ordered Mark Charter to assassinate Sharp, a renegade British agent who defected to the enemy state of Abarain. But now the two countries are at peace and it is Sharp who has come to broker an important arms deal in Britain on behalf of his government. Charter, supposedly dead but actually in jail, is now free and is coming after Sharp, giving Steed the unusual task of protecting an erstwhile traitor.
Agent Vinkel is the latest spy from 'the other side' to get polished off and, as ever, Steed is implicated but he persuades the opposition it would be best for the two camps to team up and expose a third party, committing the murders for their own agenda. Mrs. Gale is paired with Russian Martin, who, alas, gets killed at the dentist's. Steed and his partner, Kim Lawrence - actually an actress employed by the other side to test him - follow a trail to a charm school for English gentlemen - in reality an academy for assassins.
Stefan Veliko, a young Russian pianist, is making his concert debut in London. However, he is being used as a pawn in a game to wreck Anglo-Soviet relationships. First a young woman is found dead in his apartment, which is exploited to blackmail him into killing a delegate during a piano recital. Fortunately, he has the Avengers on his side, although it is a close call for Mrs. Gale.
Young Highland Guardsman Corporal Craig is found shot, his corpse containing bullets from three separate guns. He has been shot by a firing squad, for treason, and when Steed re-enlists in the Army under his old rank of Major, he could very well end up the same way, thanks to a bizarre plot by latter day Jacobites, whose plan is to overthrow the monarchy and crown a descendant of the Stuart dynasty, a descendant who just happens to be Mrs. Gale!
In investigating the death of a colleague who has died in a fire Steed and Mrs. Gale find an ornately-carved chess piece in the dead man's possession. This leads them to an importation business. On the face of it the commodity is lobsters but it is the front for a gang trying to smuggle heroin into the country.