Bernard Milan, the second-in-command of France's Counter-Espionage department, is out to discredit his chief Louis Toulouse so that he can supplant him. When a French heroin smuggler who has been arrested in New York claims that the drug smuggling was a secret mission on the orders of French Counter-Espionage, the resulting bad press reflects on Toulouse, who cannot prove that Milan was responsible. In retaliation, Toulouse hatches a plot to deal with his ambitious subordinate: in a room which he knows is filled with hidden microphones, he sends his assistant Perrache to Orly airport at 9:30AM the next morning, making Milan believe that Perrache has gone to meet a master spy who will expose Milan's treachery. However, Toulouse secretly instructs Perrache to choose someone at random from the crowd of travelers arriving at that time. After considering several possibilities from the flight arriving at the specified time, Perrache selects François Perrin, an unsuspecting violinist, who is noticeable because, as the result of a practical joke played on him by his fellow orchestra members, he has arrived wearing a black shoe on one foot and a reddish-brown one on the other. Milan takes the bait and immediately begins a series of attempts to find out what Perrin knows—never ever realizing the fact that Perrin knows nothing at all about espionage. Milan's machinations involve Perrin in a series of increasingly peculiar adventures which he either avoids or escapes from by pure luck, and although Perrin is largely oblivious to the mayhem occurring around him he can't help noticing Milan's top agent, the beautiful femme fatale Christine. Adding to the confusion is the fact that Perrin is having an affair with Paulette Lefebvre, the wife of his best friend Maurice, and Maurice, upon accidentally hearing a recording of Perrin and Paulette having torrid sex, jumps to the mistaken conclusion that Paulette is having an affair with a florist. All the time, Toulouse and Perrache watch the chaos serenely, although Perrache is troubled by his chief's callousness toward the risk that Perrin might be killed. In a startling scene, Christine greets Perrin at her apartment's front door in a demure high-necked black-velvet dress, then turns around and shows that the dress is backless, displaying discreet buttock cleavage. A slapstick love scene ensues, concluding with Milan's decision to have Perrin eliminated. More mayhem and treachery follow, climaxing in the deaths of not only agents from both Toulouse's and Milan's groups but also Milan himself, who only learns the truth about Perrin from Perrache just before he dies. Realizing how he has been fooled, Milan dies with a smile of appreciation. Maurice, who has repeatedly walked in on the aftermaths of the shoot-outs in Perrin's apartment, suffers a total mental breakdown. The film ends as it began, at Orly airport. Perrin pushes a huge Louis Vuittonsteamer trunk in an airport luggage cart, talking softly to Christine, who is hidden inside. Their destination is Rio. Toulouse, who has been watching Perrin's departure on a monitor, instructs Perrache to contact Perrin when Perrin returns, remarking "After all, he handles himself pretty well."
