When Dr. David Keel's fiancée is murdered by drug smugglers, the distraught physician swears to avenge her death by bringing the criminals to justice. He's aided in his quest by a illusive character who calls himself John Steed.
Dr. Keel poses as the missing husband of one of his patients and is shot for his troubles. A murderous anesthesiologist plans to finish the job by tampering with Dr. Keel's oxygen during a minor operation.
A General fakes his own death and arranges his daughter to be kidnapped in an attempt to thwart his wife's plan to grab his entire fortune. Unfortunately, the girl falls into the wrong hands and needs to be rescued by Steed.
Dr. Keel rescues a girl who tries to commit suicide by jumping into the Thames River. He discovers that the young lady is a trapeze artist with a visiting circus and foreign agents seek to capture her in order to force her father, a defecting scientist, to return to his country behind the Iron Curtain.
An immigrant worker in England on a forged passport picks up a radioactive isotope by mistake. Unless he's located quickly, he and anyone near him will suffer from radiation poisoning. The worker thinks the police are pursuing him because of his being an illegal alien and does his best not to be captured.
Steed investigates a series of arson cases. He finds a clue that the next victim will be a hairdressing salon on the verge of bankruptcy. Carol is snooping around the salon at Steed's behest when the building explodes.
Frank Preston is released from prison with only one thing on his mind - the recovery of 100,000 pounds that he hid after his final robbery. Frank isn't the only one after his ill-gotten gains - he's trailed by two thugs, a scheming wife and John Steed.
Dr. Keel rescues a dancing instructor who nearly died from gas asphyxiation. When he returns to check on his patient, the police arrest the doctor for murder. It seems that the doctor's scarf had been used to strangle the unlucky young woman.
Sir Thomas Waller disapproves of his daughter's choice in boyfriends and hires a criminal to scare off an unwanted suitor. Keel and Steed rescue the swain from the criminal's henchman and discover a clue to an extortion plot.
While assisting hurricane victims in a remote Mexican village Dr. Keel treats a number of food-poisoning cases. He discovers that the vegetable oil the villagers have been given to cook with is actually hydraulic-fluid.