Cédric Dugimont is a struggling actor facing a series of misfortunes as he tries to carve out his niche in the entertainment industry. His most notable work thus far involves appearing in advertisements for a brand of small-sized condoms. However, his fortunes take an unexpected turn when he is offered the lead role in a superhero film titled Badman, following the tragic demise of the initial star who was killed in an accident celebrating the film's announcement.
After months of rigorous training, Cédric embarks on filming alongside the well-known actor Alain Belmont, who portrays Badman's archenemy, Le Clown. However, just after wrapping up his first day on set, Cédric receives distressing news that his father, police chief Michel Dugimont, has collapsed unexpectedly. Wearing his Badman costume under a jacket, Cédric hastily drives the Badmobile to the hospital. Unfortunately, a malfunctioning prop explosive in the trunk leads to a chaotic crash through a nearby bank.
Upon awakening, Cédric finds himself afflicted with amnesia but soon becomes convinced he is Badman after seeing film props in his car. This misbelief leads him to attack two police officers near a lake before he makes a sudden escape. During this tumultuous time, he is taken in by a family who offers him shelter for the night. Intrigued by the plot, he activates a prop watch that plays a recorded scene of Le Clown holding Badman's wife and child hostage, further entrenching him in this fictional world.
Determined to confront the supposed villain, Cédric embarks on a mission to Le Clown's hideout in Vaucresson, which he discovers to be Belmont's actual residence. Along the way, Cédric gets a ride from Laure Laville, a news reporter, but they are sidetracked when they encounter a bank heist perpetrated by the notorious Le Schizo and his gang. When one of Le Schizo's henchmen attempts to abduct Laure, Cédric instinctively taps into his superhero persona to subdue the criminal, after which he drives off with Laure, only to leave her behind moments later.
As Cédric finds himself on the run from Le Schizo and his crew, he sneaks into Belmont's mansion, where he stumbles upon a birthday celebration. It is here that he realizes that the supposed captives of Badman's family are, in fact, Belmont's actual wife and son. In a misguided twist of fate, Cédric kidnaps Belmont's son, seeking refuge in a barn, but soon finds himself knocked unconscious by his father Michel. Michel, upon recognizing his son, allows Cédric to escape to safety.
With the help of his sister Éléonore and friends Adam and Seb, Cédric is facilitated back to the studio where they desperately try to rekindle his memories. Just as Cédric has a breakthrough in recollection, Le Schizo confronts him about the failed robbery. In a riveting showdown, Cédric uses a grappling hook to disarm Le Schizo, inadvertently sending his gun flying into the adjacent studio. After an intense battle, Cédric manages to subdue Le Schizo and leaves him for Michel and the police to take into custody.
The following day, Cédric visits Laure and discovers that she is his former girlfriend, leading to a heartfelt reconnection. In an odd twist, the news mentions that Tom Cruise is accidentally shot during the filming of a Mission: Impossible sequel. As the world premiere of Badman unfolds, Cédric acknowledges his father's retirement from active duty as a police officer, concluding with a triumphant scene from the film where Badman overcomes Le Clown and rescues his family.
