The film is set in 1910; the story beginning by documenting the flooding of the River Seine that year.
Shy projectionist Emile has a passion for film and is in love with his co-worker at the cinema, Maud. His friend, an exuberant inventor and delivery driver, Raoul, picks him up from work to transport him in his bizarre vehicle, called ”Catherine”, to obtain a new belt for his projector. In purchasing a new belt, Emile also buys himself a new camera, which is almost stolen by a thief. The story also introduces Lucille, Raoul’s childhood friend: a cabaret singer at the club L’Oiseau Rare, ”The Rare Bird”, whose aunt Carlotta tries to marry her to the wealthy Police Commissioner, Victor Maynott. One evening, Raoul brings Emile to make a delivery to the Botanical Gardens. In the absence of the Professor who works there, the place is guarded by his assistant, a proboscis monkey named Charles. Here, Raoul experiments with an ”Atomize-a-Tune” mixture which temporarily gives Charles the voice of an opera singer and an unstable ”super fertilizer” which instantly grows a sunflower seed into a giant sunflower, which topples towards Raoul and Emile. In the ensuing disorder, an explosion occurs and the two chemicals are mixed. Everyone is unscathed, but Emile is convinced he has glimpsed a monstrous creature, a photo of which later appears in the newspapers.
An investigation is launched into the whereabouts of the creature by Maynott’s second in command, Paté, but is fronted by Maynott, in the hope of popular support for his mayoral candidacy. At the same time he tries unsuccessfully to seduce Lucille. Meanwhile, Lucille is trying to find a new musician for her show, and turns down the cabaret’s waiter, Albert. Trying to vacate the cabaret, Albert stumbles across the creature and flees, terrified. Upon seeing the creature, Lucille is frightened, but hears it sing and discovers it is not dangerous but has a lovely singing voice. She therefore welcomes it, under the name of Francœur, meaning ”honest heart”, . It is then revealed that the creature is a flea that the fertilizer, amidst all the other chemicals in the laboratory explosion, enlarged to human scale. During the investigation, Emile and Raoul’s role in the laboratory incident is discovered. For Maynott’s interest in the creature, they receive the Medal of Honor. On a challenge set earlier by Lucille, Raoul uses this to get the best seats at Lucille’s show at the ‘Rare Bird’, where Francœur and Lucille sing as a duet. After the show, Emile and Raoul congratulate Lucille on her show, but Lucille reveals the identity of Francœur, which Albert reports to the police. Emile, Raoul, and Francœur narrowly escape and Albert is arrested for lying to the police.
The following day, Maynott opens the Montmartre Funicular, which serves Montmartre and the Basilica of the Sacré Cœur. The trio, along with Francœur and Charles, decide to feign the death of the creature. When Maynott discovers the creature hiding under the stage, Francœur and his friends are chased through the streets of Paris by Maynott. The chase concludes at the tip of the Eiffel Tower. After a battle to protect Francœur from Maynott, a gunshot from Maynott and Francœur’s sudden disappearance leads everyone to believe he is dead. Maynott is then arrested by Paté on the basis that Francœur is innocent, while Emile and Maud fall into each other’s arms. Later that evening Lucille is distraught by the ”death” of Francœur; but Raoul convinces her to sing anyway. Whilst struggling to begin, she hears Francœur, restored to his natural size, sing in her ear. Some time later, the absent Professor returns from his trip; and when the three friends explain the situation, he permanently returns Francœur to human size. Francœur receives second billing on the posters advertising Lucille’s show. Lucille and Raoul later share their first kiss in Lucille’s dressing room.
In a mid-credits scene, Raoul, Lucille, Francœur, Maud, Emile, Charles, Carlotta, and Paté scatter super-fertilized sunflower seeds to drain the flooded Seine. In a second post-credits scene, Maynott is shown in the same cell as Albert and the thief from earlier, forced to endure the duo’s appalling singing.