Having been rendered unconscious by knock-out gas in his pursuit of rogue agent Jimmy Merlin, Steed awakes to find that London is deserted and under martial law, headed by Brigadier Hansing. Under cover of a supposed evacuation, the Brigadier, incensed that he is to be retired from the army, has planted an atom bomb to destroy the capital. All Steed has to do is find it.
With whole weeks missing from his life and evidence of actions that he simply cannot remember, Steed finds himself in an endless series of 'deja vu' situations. In reality he is slowly being hypnotised and the aim is to mesmerise him into killing 'Mother'.
To celebrate his birthday 'Mother' goes to visit two elderly aunts to whom he tells a story from the Avengers' case files. It involves a criminal caper to steal art treasures. The treasures were to be held for safe keeping in an underground vault in the event of a national emergency but the criminals aimed to squirrel them away once in the vault. It is a hair colour changing story indeed.
The vicar of a Norman church asks the Avengers' help when a team of archaeologists excavating a site underneath the church are all killed and their bodies rendered completely devoid of any energy. This is due to a little black box which sends out deadly charges of electricity. Furthermore its charges are nurturing a race of thingumajigs, pure energy creatures which will eat anything and must be stopped before they do great harm.
Following a tip-off Steed and Miss King are present when a member of the Acme Precision Combine is stabbed, seemingly by a sleep-walker who has no recollection of the murder. This leads the Avengers to a clinic run by Dr. A. Jaeger who may or may not be programming deadly sleep-walkers.
Coming home from a costume party Miranda Loxton witnessed a killing by a member of Murder International and Steed is assigned to protect her until she gives evidence at the trial. Only Miss King knows that he has taken her to a boyhood haunt but then she is caught and wakes up in a sinister hospital where she is pumped for information about Miranda's hideaway.
Three men and a woman called Circe politely call on the country home of Steed's friends, married couple Bill and Laura Bassett. Unfortunately their visit turns sinister as they plan to implant explosives into the throats of the couple and send them off to destroy a nearby peace conference. Happily for the Bassetts - and the conference - Steed is coming to visit for the weekend, and he knows what to do.
Miss King is abducted whilst visiting an antique shop and wakes up in a room beautifully furnished in a style from fifty years earlier. She is addressed as Pandora. Steed follows a lead which takes him to retired World War One agent Lasindell, the love of whose life was called Pandora and bore a strong resemblance to Miss King. It is a plot by his greedy relations to make him divulge the location of his fortune or else Pandora, alias Miss King, comes to harm.
Found wandering in a snowy field Helen Pritchard claims to have seen a man who was supposed to have been a corpse but was very much alive. This leads Steed to investigate the Happy Meadows funeral parlour and the cheery Mr. Bagpipes Happychap. Unfortunately the undertakers seem unable to keep their 'stiffs' from walking and Steed has to arrange his own funeral in order to find out more, though ultimately he and Miss King do get a send-off that is out of this world!