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, Pâté, 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 on suspicion of lying to the police.