The story focuses on Madame Souza, an elderly woman raising her young grandson Champion. Souza notices her grandson is sad and lonely so she buys him a puppy named Bruno to cheer him up. Although initially happy he quickly becomes melancholy once again. After discovering that Champion has a keen interest in road bicycle racing, she buys him a tricycle. Years later, Champion has become a professional cyclist with Souza as his coach.
Eventually, Champion enters 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 along with Bruno follow the men but lose their trail soon after reaching Belleville. Lost in the city with no way to find Champion, Souza has a chance encounter with the renowned Belleville triplets, music hall singers from the 1930s, now elderly women turned improvisational musicians. The sisters take Souza in to stay with them and over time she becomes a part of their group. Meanwhile, the mafia boss has a mechanic build a stationary cycling machine for the kidnapped cyclists to race on to create their own mini Tour de France for gambling.
At a fancy restaurant the triplets plus Souza perform a jam session using a newspaper, refrigerator, vacuum and bicycle wheel. The mafia boss who kidnapped her grandson happens to be in the same restaurant, and, with the help of Bruno, Souza realizes he has Champion. She tails one of the mafia's minions and discovers the scheme. That night, several mafia bosses and their henchmen arrive at the mafia hideout and place bets on the riders. Madam Souza, Bruno and the triplets infiltrate the hideout and sabotage the contraption, unbolting it from the ground and turning it into a pedal-powered vehicle on which they all escape. The mob henchmen pursue them, but each of them are disposed by Souza and the triplets. The film ends with Souza, Champion, Bruno, the triplets riding on the vehicle out of Belleville.