The film begins with a flashback showing The Triplets of Belleville: a trio of singers performing on stage in the 1920s (dancing alongside other celebrities including Josephine Baker and Django Reinhardt).
The story focuses on Madame Souza, an elderly woman raising her young charge, her grandson, Champion. Souza notices his listlessness that is attributed to hints of his being orphaned. When she notices his (brief) interest in music, she tries to get him to play the piano. When this fails to inspire, she buys him Bruno, a dog. After a time he becomes melancholic once more. She discovers Champion’s keen interest in road bicycle racing (a photo on his wall shows his deceased parents with a cycle), she buys him a tricycle. We see them years later, as Champion the cyclist and Souza, his coach.
Champion competes the Tour de France but during the race, he and two other riders are kidnapped by two French mafia henchmen and brought to the bustling metropolis of Belleville.
Souza and Bruno follow the men, but lose their trail. Lost and with no way to find Champion, Souza has a chance encounter with the renowned Belleville triplets, music hall singers from the 1920s, now elderly women turned improvisational musicians. The sisters take Souza to their home and she soon becomes a part of their group. We then see the wine mafia boss using the kidnapped cyclists as horses in a stationary cycling machine. They are raced in a simulated Tour de France race for gambling.
At a fancy restaurant, the Triplets and Souza perform a jam session using newspaper, refrigerator, vacuum cleaner and bicycle wheel as instruments. The wine mafia boss happens to be in the restaurant and, with the help of Bruno, Souza realizes he has Champion. She tails one of the minions and discovers the scheme. That night, mob bosses and henchmen arrive at the hideout to bet on the riders (one of whom falls off his bicycle from exhaustion, at which point the bookmaker shoots him as one would an injured horse). Madame Souza, Bruno and the Triplets then infiltrate the hideout, sabotage the contraption, free it and turn it into a pedal-powered vehicle on which they all escape. The mob henchmen pursue them, but are all overcome. The film ends with the motley group riding out of Belleville, and a flashforward to an elderly Champion reflecting on the adventure, as told to him by the then-deceased Souza.