François Voisin is a telecommunications employee with an ostensibly ordinary life. In reality, François is a secret agent. He is reputed to be one of the best in his profession, earning him the monikerl'Épée or, "The Sword". He hides his activities from his wife.
François is coming home on his 40th birthday, for which his wife has secretly prepared a surprise party with old friends with whom he used to fence. He is called back by his colleague Albert for a mission involving planting a microphone in an arms smuggler's car. He succeeds after a struggle. As he arrives home, his friends mock his "boring" life.
The planted microphone leads the Secret Service to a prostitute. After infiltrating her apartment and planting a camera, they discover she is helping a missile expert and the smuggler to meet. They then intercept and destroy a missile convoy. Meanwhile, François' wife, Hélène, is living a monotonous life.
Hélène then meets Simon, who pretends to be a secret agent while, in reality, he is a car dealer living in a caravan. François discovers Hélène is meeting someone in secrecy and believes she is cheating. He, therefore, uses the agency's resources to spy on her and find out more about Simon. When Simon lures Hélène to his caravan, François kidnaps both of them. He makes his wife believe that Simon is a terrorist and then makes Hélène believe she has to work for the agency to secure her freedom. She is unaware that François is behind this, and gets sent to a hotel room for a mission where he plans on surprising her. At this point, they both get kidnapped by henchmen of the arms dealer. After François admits his true identity to Hélène, he manages for them to escape. They thwart the arms dealer's plan of blowing up a football stadium in Paris and all ends well.
The film concludes with François' 41st birthday. He and Hélène kidnap a dangerous man who turns out to be Simon, pretending to be someone else.
