In the first shot, a quintessentially British man in a dark suit and hat, tightly rolled umbrella, and immobile face, strides along Havana streets besieged by grinning musicians and hawkers. Hawthorne (Noel Coward) is a Secret Service official based in Jamaica charged with recruiting a group of secret agents to report on Cuban military activities for a monthly stipend. Hawthorne approaches Jim Wormold (Alec Guinness), an expatriate Englishman living in Havana in the late 1950s with his beautiful teenage daughter Milly (Jo Morrow), who is attending a Catholic all girls school. Jim has lived in Cuba for fifteen years, and despite the rocky political climate, considers it home. Wormold's spare time consists of drinks with German Dr. Hasselbacher (Burl Ives) and fawning over his daughter, who has reached that precarious threshold between childhood and adulthood.Milly is being trailed by police captain Segura (Ernie Kovacs), who is friendly with the nuns at Millys school, and is looking for a traditional and pretty young woman to be his wife from among the graduating students. Milly, a dazzling blonde, is his target. When the very conspicuous Hawthorne shows up, Segura guesses that something of interest to the police may be going on, and makes inquiries hoping to use the information for blackmailing Jim in his favor, if necessary.Jim owns a vacuum cleaner shop but isn't successful enough, as he wants to provide greater luxuries for Milly, such as finishing school in Switzerland, and Milly has some expensive expectations of her own, such as owning a horse.Jim says yes to Hawthorne because of his money needs, but he has no clue about spying. Hawthorne shows him how to encode messages for telegraphing his reports and gives him general pointers. Hawthorne just tells him to approach people he knows, but that gets Jim nowhere. Jim approaches one of his trusted employees but the man misunderstands and thinks Jim is asking for prostitutes. Several times during the rest of the movie, a prostitute is brought to him by this employee.Prodded by Millys wishes to have a horse, Jim gets the Secret Service to pay for his membership in a hugely expensive country club, with the excuse that he needs access to a place where he meets important people. When he approaches members of the country club, his social ineptitude creates confusions and they think he is trying to come onto them. Instead of spying, Wormold spends most of his time drinking at the country club, which he can finally afford.Jims closest friend, a former German army officer, Dr. Hasselbacher, suggests that the best secrets are known to no one, so Jim decides to pretend to have a list of agents and provide fictional tales for the benefit of the bosses in London. Hasselbacher opines that Jim would not be doing anything immoral because countries feel obligated to play a game and spy on each other, and it might has well be him who gets paid.As he does nothing real to report, Jim pretends first to recruit agents and then that they discover secret constructions. In order to have a semblance of credibility, the names and professions he gives his fake agents are names and professions of real people.When London messages demand some information beyond lists of names, Jim notices that images of his vacuum cleaners, if enlarged and doctored, look like odd buildings having a scientific or military purpose. He sends drawings to England, and the spy boss C (Ralph Richardson) is deeply impressed. Some other reports are inspired by comic strips. He is seen as the best agent in the Western Hemisphere, and the spy agency decides to send him support staff.Soon Beatrice Severn (Maureen O'Hara) arrives in Havana, sent by C to be his "bookkeeper" and assistant. Beatrice finds Jim romantically attractive. When she begins to catch on to the deceptions, she keeps quiet despite misgivings.There are others who get suspicious of Wormold at the Agency. One agent comments to C that the drawings look like parts of a vacuum cleaner, enlarged. Hawthorne hears this while he is behind C, and realizes what Guinness is sending, but he keeps quiet.Captain Segura is aware, after hearing of Jims approaches to local fellows, that Jim was recruiting spies, but he bides his time, as his interest is getting Jim's consent for him to marry Milly. Milly accepts some courting, but her interests are rather with her horse rather than marrying.Dr. Hasselbacher, tempted by Jims reports of easy money in exchange for false information, also starts to sell lies, judging that no significant harm results from the deception.Wormolds originally harmless fraud eventually becomes dangerous. As British Intelligence agencies begin to take his work seriously, so does the other side, and thus Wormold is a wanted man, dead or alive.Spys from the other side begin to decode his cables and start disrupting his network. One of the men whom Wormold pretended was one of his agents dies in a suspicious accident. An engineer named Cifuentes (Grégoire Aslan) whom Jim approached multiple times at the country club is kidnapped and dropped off at Jims doorstep bound and gagged. These events give London the impression that Wormold's networks are under attack.Hawthorne has Jim go to Jamaica for consultations, where he warns him that Jim's discovery of military secrets is so impressive that the other sides agents have decided to silence him by assassination, most likely by poisoning at a banquet Jim has to attend as part of his vacuum cleaner business. Hawthorne instructs Jim in a variety of techniques to avoid being poisoned by food or drink.Jim manages to figure out the assassin is none other than a friendly man he met on the flight to Kingston, but he is unable to prevent the murder of his friend Dr. Hasselbacher.In seeking to avenge his friend, Jim arranges to have a meeting with Segura, ostensibly to discuss the possible courtship of Milly. Jim must get Segura so drunk that he passes out, so he can steal a loaded gun. As both of them are avid checkers players, he arranges a game of checkers to be played with miniature bottles of liquor. Each time one is captured, it must be opened and drunk immediately. As the captain is in fact a better player, the Captain gets totally, drunk, and Jim takes the gun to kill the would be assassin.Pretending friendship, he has made arrangements to go out on the town with the assassin, using himself as a decoy, assuming that the assassin will use the occasion to have him ambushed and killed by others somewhere. He tells the assassin they will be going to a couple of semi clandestine night clubs, giving their names, but in fact he goes to others, so the ambushers are sent to the wrong place, the assassin is isolated, and Jim shoots him with Seguras gun at the door of a brothel.Captain Segura wakes from his drunken binge unaware of what happened. The next day, however, after Jim and Milly attend Hasselbachers funeral, the captain orders Jim deported, and he must return to London.He tells all details of what he has done to Beatrice, who decides to quit the spy business and return to London with him. Meantime, Beatrice and Milly have become friends, Milly accepts Beatrice as a future stepmother.Accompanied by Beatrice and Milly, Jim goes to a meeting with C and other spy bosses in London. While he is waiting to be seen, the officials discuss what they should do with him. To reveal to the Prime Minister and the other top brass that Jim concocted all his intelligence would have a damaging effect on the intelligence aparatus, so they come up with a solution. A story is fabricated, claiming the hardware that Jim's agent had seen had since been dismantled. Wormold is told he is to receive an O.B.E, and the secret service offers him a position teaching espionage classes to new recruits in London. With this money, he can afford to send Milly to the fancy school in Switzerland.