Pierre Brochant is a Parisian publisher who regularly takes part in a peculiar tradition known as the "idiots' dinner." At these gatherings, elite, fashionable businessmen must bring along an unsuspecting "idiot" for the evening's entertainment, allowing the guests to ridicule this oblivious person without their knowledge. The aim is to choose the evening's "champion idiot" from among the gathered guests.
With the assistance of a friend known for spotting these "idiots," Brochant discovers an unexpected treasure in François Pignon, a lively employee from the Finance Ministry, a position Brochant despises due to his own tax malfeasance. Pignon has an unusual obsession with constructing detailed matchstick models of iconic landmarks. Just as Brochant extends an invitation to Pignon to join the dinner, he finds himself suddenly incapacitated with excruciating back pain while golfing at his posh country club. Compounding his troubles, his wife, Christine, leaves him after realizing he still intends to host the dinner, and turmoil arises when Pignon arrives at Brochant's apartment.
At first, Brochant wishes for Pignon to leave, but as the evening unfolds, he increasingly depends on him—thanks to his back issue and complications in his personal life. Brochant seeks Pignon's help in making phone calls to track down Christine, yet Pignon's well-meaning efforts frequently end in disaster. He unwittingly discloses Brochant's secret affair with Marlene Sasseur to Christine, mistakenly thinking Marlene is Brochant’s sister due to her name, and even invites tax inspector Lucien Cheval to Brochant's residence, forcing Brochant to hurriedly stow away his valuables to conceal his tax fraud.
Throughout the chaos, Brochant attempts to mends fences with Juste Leblanc, an old friend from whom he once wrongfully took Christine. As the evening progresses, Brochant is compelled to reflect on his past missteps. Pignon, in a moment of honesty, calls Christine to share all the efforts Brochant has made in trying to redeem himself, including ending his affair and seeking reconciliation. However, when Christine becomes skeptical and questions Pignon about Brochant’s presence during the call, Pignon lies, saying he's alone in a phone booth.
In a tragic twist, when a softened Christine reaches out to Brochant for a conversation about their future, Pignon answers the call instead, leading her to believe that Brochant has manipulated the situation. The film concludes with Brochant feeling exasperated and blaming Pignon for the chaos that has ensued in his life.
