1952. Married couple Tommy and Tuppence have a chance encounter on a Paris train with an agitated young woman, named Jane Finn, who suddenly disappears and never returns to her seat. Back in England, Tommy is focussed on their fledgling honey business, Tuppence is much more concerned about the missing girl and her investigations bring them into contact with Tommy’s uncle, and Third Floor bigwig, Major Anthony Carter. It turns out that Jane Finn was carrying a secret recording that would potentially reveal the identity of a legendary Soviet assassin, known only as Mr Brown, who they believe will strike soon in Britain. Carter believes Brown’s cronies must have kidnapped Jane - they can only hope she managed to hide the recording before they got to her. Despite Carter’s attempts to keep them out 54 of it, Tommy and Tuppence soon find themselves compelled to infiltrate Brown’s gang in the hopes of finding Jane Finn, the recording and foiling Brown’s next hit.

With Tommy mistaken for a criminal mastermind, Drennan, due to join Brown’s gang from the north, and Tuppence employed as a maid to a faded soprano, Rita Vandemeyer, with suspicious connections to Brown, the couples’ investigations into Brown get deeper and more dangerous. Tommy tries to escape the pleasures of the gang’s seedy Soho den long enough to search for Jane within its dark shadowy corners and back rooms, but he is caught and must attempt to return tomorrow to try again…except this time the gang will be expecting the £1,000 Drennan was due to bring. Tuppence feels she is getting closer to the assassination plot when a mysterious package is delivered to Rita from Brown and locked in a secret room. She is also shocked to discover that one of Rita’s closest friends, James Peel KC, is in fact an agent of Carter’s on the same case. In the face of ever increasing peril and Carter’s continued obstruction, Tommy and Tuppence must somehow find the funds to allow Tommy’s re-entry to the Soho den, solve the mystery of Rita’s secret package and, for their own safety, try to maintain their cover.

Tuppence ponders the cryptic message she discovered in the Soho den at the end of Episode Two, and worries about Tommy who, with his cover blown, is being driven to a remote location by Brown’s gang. Tommy’s given a job to do for Brown on pain of the death of his son, George. He must steal a specific file from under Major Carter’s nose on the Third Floor, he succeeds, with Tuppence’s help, but they redeem themselves by solving the mystery of the location of the secret recording that will identify Brown. But before they can listen to it, it’s ripped from their grasp by Jane Finn’s uncle Julius, who had appeared their friend until now. When Julius claims to be Brown, our heroes smell a patsy and must penetrate Julius’ lies to reveal Brown’s identity, find out where he’s keeping Jane Finn, and foil the potential assassination of a visiting American dignitary of incredible importance to the British government.

In N or M? Major Carter commandeers Tommy for a mission he must keep completely secret, even from Tuppence. There is a leak on the Third Floor and Tommy is the only man Carter can trust. A British scientist, Gilbert Worthing, who was working on a top-secret prototype for a nuclear bomb, has gone missing from a military base in Cromer, along with his work. All Tommy knows is that the perpetrator is staying at the nearby Sans Souci guesthouse and is most likely a Soviet super spy code named N or is it M? Tommy must travel in disguise as a birdwatcher to Cromer, in order to identify which guest at the Sans Souci is holding the British government to ransom. Things get even more complicated when it turns out that Tuppence, dismayed at being left out, has beaten him to it and already ingratiated herself with the other guests. But Tommy and Tuppence must learn to work together again if they are going to identify the Soviet spy, save the missing scientist, and discover the location of the missing bomb before half of Norfolk is obliterated.

Tommy and Tuppence think they are getting somewhere until their first major suspect Major Khan is killed at the local Veterans Ball. It turns out Khan was looking for N too, for a different reason, and N got to him first. And so Tommy and Tuppence turn their attention to their fellow occupants of the Sans Souci to find the potential nuclear bomb thief. Tommy has his eyes on Carl Denim, a shifty young beatnik who loves to flirt (especially with Tuppence) and is spending a lot of unexplained time around the coast, whereas Tuppence is convinced the culprit is Mrs Sprot, a glamorous and sensual woman, apparently married, but traveling alone. She was certainly in Khan’s room the night of his death, but was it to cover up their affair, or the fact that she is N? There are so many potential suspects and dead ends. Tommy and Tuppence can’t help but get their wires crossed and let their guard down. Tommy inadvertently ends up in the lion’s den. He finds himself in the home of an apparently kindly old sea dog, Commander Haydock, who is revealed to be not what he seems to be.

Tommy has finally found the missing scientist, Gilbert Worthing, but they are both being held against their will at Commander Haydock’s, and we’re still none the wiser as to who N really is. It turns out that Gilbert managed to hide a vital key to his device before his capture, and that without it the bomb N and Haydock have stolen is entirely useless. So when Tuppence storms Haydock’s house and helps Tommy and Gilbert to escape, it seems like the only thing left to do is retrieve the key from the strange place Gilbert hid it. The only problem is that it’s in the Sans Souci, where N is lying in wait. When N gains possession of the key Tommy and Tuppence find themselves once again backed into a corner. With Carter’s help they must come up with their most ambitious and risky plan yet to save the lives of thousands of innocent British citizens.