Bernard Milan, Bernard Blier, the ambitious second-in-command of France's Counter-Espionage, hatches a plan to discredit his chief, Colonel Louis Toulouse, Jean Rochefort, so he can seize control. When a French heroin smuggler arrested in New York claims the drug operation was a secret mission ordered by Counter-Espionage (actually Milan's orders), the resulting press backlash falls squarely on Toulouse, who cannot prove Milan's treachery.
To snare Milan, Toulouse lays a careful trap: in a room filled with hidden microphones, he sends his assistant Perrache, Paul Le Person, to Orly Airport at 9:30 AM the next morning and instructs him to pick a random traveler from the arriving crowd, suggesting that a master spy will expose Milan. The ruse hinges on Perrache's random choice, a move designed to make Milan believe Perrache has connected him to a powerful ally.
From the crowd, Perrache selects François Perrin, an unsuspecting violinist, Pierre Richard, who arrives with a striking oddity—a black shoe on one foot and a reddish-brown one on the other—an inside joke among his fellow orchestra members. Milan, listening in, launches a sequence of relentless attempts to pry Perrin's knowledge, never realizing that Perrin knows almost nothing about espionage, though he carries deep expertise in music.
Amid the growing confusion, Perrin becomes aware of a captivating presence: Christine, a femme fatale portrayed by Mireille Darc. Milan orders Christine to seduce Perrin, and she greets him at her apartment door in a demure black-velvet dress that, once she turns, reveals a backless silhouette with discreet buttock cleavage. Their intimate encounter unfolds as Milan watches on a monitor, and Milan, despite Christine's belief that Perrin could not be a spy, resolves to have Perrin eliminated to cover his tracks. Perrache, meanwhile, defies orders to recall their agents to save the innocent Perrin.
Chaos deepens as Maurice Lefebvre, [Jean Carmet], Perrin's best friend, discovers the truth about Perrin's affair with Paulette Lefebvre, and Christine begins to defect from Milan's circle to protect Perrin—and she finds herself drawn to him. Paulette Lefebvre, [Colette Castel], is married to Maurice, and the complicating triangle becomes a focal point as a recording of Perrin and Paulette—made inside a floral delivery truck by Milan's agents—spreads misreadings of loyalty and betrayal.
The story crescendos with a cascade of violence: agents from both Toulouse's and Milan's factions fall, and Milan himself dies only after Perrache reveals the truth to him at the last moment. Maurice, who repeatedly stumbles upon the aftermath of the shoot-outs in Perrin's apartment, experiences a complete mental unraveling.
At Orly Airport, Perrin wheels a colossal Louis Vuitton steamer trunk on a luggage cart, with Christine hidden inside, as they head toward Rio de Janeiro. Toulouse, watching the departure on a monitor, instructs Perrache to contact Perrin when he returns, concluding with a knowing smile: “After all, he handles himself pretty well.”
