France, 1970. The Yé-Yé age is at the top and everything is joy and happiness. However, the lives of Baron Claude Tranchant and his wife Baroness Élaine Tranchant are shocked after a Baron's close friend has an accident while he drives Baroness' car, discovering that the brakes had been cut. Convinced that someone is trying to kill Éliane, Claude calls his close friend and politician Jacques Chirac, who calls Commissar Maurice Lefranc for sending the best police of Paris to find the killer. When Lefranc asks for his detectives, he discovers that the only one available is the absent-minded, incredibly clumsy and absolutely disastrous Jean Boullin. Reluctant, Lefranc gives Boullin the case, he travels to the Baron's summer house in the French Riviera where each year the Baron organizes a party for him and his wife's relatives. After to know that Élaine has received death threat letters, Boullin tells Claude his plan: asking Claude not reveal to the rest the truth about the car crash and death threat letters, he will work disguised as a waiter of the staff, hearing any conversation of the guests that reveal to the killer. But each attempt to discover the killer ends of the worst way time and time again, causing not only the anger of Claude and the others, but Lefranc. Will can Boullin solve the case and save Élaine before his non-existent abilities for investigation cause more damage than he can handle?