The main character is Bazil (Dany Boon), whose father was killed attempting to defuse a land mine in the Western Sahara when Bazil was a child. The film begins with his father's death and then jumps 30 years later to Bazil working in a video rental shop in Paris. Bazil is watching The Big Sleep on a small television. The ending of The Big Sleep segues into the opening credits of Micmacs, shot in the old style in black and white with extended production credits.
Bazil hears gunfire and cars outside, opens the door of his shop, and is hit in the head by a stray bullet. A surgical team discusses whether or not to remove the bullet from his brain. Removing the bullet will risk damaging his brain further, while leaving it in will leave Bazil mostly healthy with the risk of dying suddenly at any moment. The head surgeon flips a coin and decides not to remove the bullet. Bazil returns to his job to find that he has been replaced. As he leaves, his replacement gives him a shell casing that she found from the bullet that had struck him.
Bazil becomes homeless and lives on the streets of Paris for two months, before being "adopted" by a man named Slammer (Jean-Pierre Marielle). Slammer brings Bazil to a cave carved in a trash dump, where a group of scavengers live. Bazil meets several new friends: Elastic Girl (Julie Ferrier) is a contortionist, Mama Chow (Yolande Moreau) is a cook and leader of the crew, Remington (Omar Sy) is a former ethnographer who speaks in old-fashioned cliches, Buster (Dominique Pinon) is a human cannonball, Tiny Pete (Michel Crémadès) is an artist who designs moving sculptures from scavenged trash, Slammer is a former convict and guillotine survivor, and Calculator (Marie–Julie Baup) is a young woman who measures and calculates things with a glance.
While scavenging for trash, Bazil discovers two office buildings and factories on opposite sides of a street. One is the arms manufacturer who built the land mine that killed Bazil's father, and the other is the manufacturer who made the bullet that lodged in Bazil's brain. Bazil decides to go inside, flips a coin, and chooses one of the offices. He asks to speak with the CEO, Nicolas Thibault de Fenouillet, but is immediately thrown out of the building. He crosses the street and manages to hear a speech by the other CEO, François Marconi.
Bazil follows Marconi home and hangs a microphone down his chimney. He hears a phone conversation arranging a meeting between Marconi and associates of Omar Boulounga, an African dictator seeking arms for an upcoming violent conflict. Mama Chow's crew decides to help Bazil exact revenge on the two arms dealers. They first incapacitate Boulounga's men by planting drugs on them in an airport.
Remington, claiming to be Boulounga's right-hand man, meets with De Fenouillet and proposes the same deal which was offered to Marconi. Later, Remington calls each of Marconi and De Fennouillet and angrily cancels the deal. He tells Marconi that he will be dealing with De Fennouillet, and tells De Fennouillet that he will be dealing with Marconi. The two CEOs are furious and declare war on each other. Bazil and his friends break into Marconi's house and steal and replace his luxury cars, and steal De Fenouillet's collection of body-part relics from historical persons. They steal and destroy a truck full of bombs from Marconi's plant. Marconi assumes that De Fennouillet is responsible, and arranges to sabotage a machine causing a massive explosion in De Fennouillet's factory. Next, Elastic Girl breaks into Marconi's apartment searching for blackmail material while Bazil waits and listens on the roof.
Marconi arrives unexpectedly, and Elastic Girl is forced to hide for hours in the refrigerator. De Fenouillet sends an armed team to attack Marconi, but Boulounga's men arrive first and take him hostage. Boulounga's men are about to execute Marconi when they are shot by De Fennouillet's men.
Marconi and De Fenouillet discuss and learn that they have been pitted against each other by Bazil. They capture Bazil on the roof, and take him away in their car. Elastic Girl comes out of her hiding place, and calls in the rest of the crew to rescue Bazil. After a car chase through Paris, Bazil is saved and Marconi and De Fennouillet are captured. The two CEOs are bound and hooded, and they hear a long plane flight followed by a ride in a car. When they are allowed to see again, they are in the middle of the desert. In a scene inspired by Sergio Leone's Once Upon a Time in the West, De Fennouillet is sitting on Marconi's shoulders with a live grenade in his mouth, while Marconi stands on a live land mine. A small crowd wearing veils sit watching them holding photographs of landmine victims. The men beg for mercy and confess to their roles in supplying arms to the IRA, ETA, and Darfur combatants.
Marconi and De Fenouillet fall and discover that the grenade and mine are not armed. The small audience is revealed to be Bazil and his friends in disguise, who have been recording the event with a video camera. In a flashback inspired by Brian De Palma's Mission Impossible, we see that Bazil and his friends simulated the entire plane flight with various sound effects, and the desert setting is simply a clearing in a Paris suburb. Bazil and Calculator upload their video to YouTube, and Marconi and De Fenouillet are publicly disgraced. The film ends with a dialogue by Remington how Bazil and the Elastic Girl get together.
Jean-Pierre Jeunet originally wrote the character of Bazil for Jamel Debbouze, but Debbouze left the project after three weeks, citing artistic and financial disagreements. The role was later given to Dany Boon.
Jeunet toured arms manufacturing plants in Belgium for research when developing the film. Some dialogue was taken directly from interviews from arms dealers. The sabotaged machine in De Fenouillet's factory was modeled exactly after an actual machine in a plant which Jeunet visited.
The film is shot in several locations in and around Paris including the exterior of the Musee D'Orsay and the Crimée bridge on the Canal de l'Ourcq, where the Marcel Carné's 1946 film Gates of the Night was shot. Jeunet also filmed in several train stations, including Gare de Lyon, Gare Saint-Lazare, and the Charles De Gaulle airport train station.
Though the film contains no obvious special effects sequences, digital color manipulation is used throughout, and specific digital manipulations were used on about 350 shots. These manipulations often involved removing people and objects in backgrounds of scenes shot on Paris streets. The closeup of Dany Boon's face during Marconi's speech was out of focus when shot, but his performance was so good that Jeunet decided to digitally focus the face rather than reshooting. De Fenouillet's first appearance in his office was constructed entirely in post production from footage filmed for a different scene. Bazil, Buster, and Slammer were digitally removed from the frame, and De Fenouillet was given dialogue whose audio would synchronize exactly with the original lines filmed.
The character of Elastic Girl performs several contortions on screen which were not digital effects. Julie Ferrier, who played the character, is fairly flexible and did some of the movement herself. The difficult contortions were performed by Julia Gunthel, also known as Zlata. Jeunet and cinematographer Tetsuo Nagata had discovered Gunthel doing an erotic show in Germany.
Tiny Pete's moving sculptures were designed and built by sculptor Gilbert Peyre.
The film contains five appearances of the film's poster, usually hidden in quick shots.
The film opens with the final sequence of The Big Sleep, with the original score by Max Steiner. Steiner's score is used throughout the film, but Jeunet also required original music. The music which appears is by Raphaël Beau, an unknown school teacher with no prior professional recording or scoring experience. Beau composed music cues for various scenes. Jeunet loved his music but moved the songs to different scenes in the final version of the film.
Jeunet also decided to reference his earlier film Amélie in the shot when Bazil first lowers his microphone into a chimney. The planned scene would show Amelie and Nino in a small apartment with several crying children. Amélie star Audrey Tautou was shooting Coco Before Chanel, and was unavailable to shoot the scene. The shot was replaced by an homage to Jeunet's Delicatessen. In the final scene, Dominique Pinon sits with Marie-Laure Dougnac and plays a musical saw.
According to Box Office Mojo, Micmacs grossed $16,331,174 in the worldwide box office. The review aggregator website Rotten Tomatoes reported a 74% approval rating with an average rating of 6.6/10 based on 133 reviews. Metacritic gave the film a score of 62 out of 100, based on 31 critics.