Greene's book was a cynical look at how Intelligence forces can show a distinct lack of intelligence when confronted with mis-information, but it is Reed's nifty idea to cast it with comedic actors, though not always playing for laughs. With such as Burl Ives, Noel Coward, Ralph Richardson, Ernie Kovacs (he plays a corrupt Cuban police official straight, but it is still funny) and Alec Guinness, it seems more a comedy of errors: British ex-pat Jim Wormold (Guinness) is scraping by a living selling vacuum cleaners in Havana, while his daughter (Jo Morrow) is developing a taste for the expensive horsey set—something that could be provided by Captain Segura (Kovacks), who has an eye for the blonde girl. Wormold has other plans for her, like an expensive Swiss boarding school. But where to get the money?
He is approached one day by Hawthorne (Coward, out of place in his dark suit and bowler hat in the mid-day sun of Havana) of the British secret service—or, as he is known, Agent 29500—to set up a bureau station for the service. For Wormold, it is extra cash, an all-expenses paid membership to the exclusive country club, and a more lavish life-style, all for keeping his daughter close. All the Service wants is results, which Wormold has trouble setting up—he is, after all, only a vaccuum cleaner salesman. Soon, he starts filing bogus reports, recruiting strangers as fellow agents (without their knowledge), building his station in importance to the delight of Hawthorne and his superior 'C' (Ralph Richardson).
However, becoming an important secret agent draws attention. He is soon assigned a secretary (Maureen O'Hara) by his superiors, wanting to build him up, and targeted for assassination by his enemies, wanting to shut him up. Doesn't matter if the information he's sending is all wrong; with so many resources at his command, he's sure to dig up something sooner or later. Scrutinized from both sides, the spy-game stops being so rewarding, and turns downright dangerous.
It's all played with a bit of a wink, with great comic actors under tight rein to let the material be funny without goosing it. Definitely worth seeing for the literate script, Reed's classic direction and the fine performances. John le Carré would later use the basic subject matter for his book (and subsequent John Boorman film) The Tailor of Panama.