
Plot:
The film is set in 1910. The story begins by documenting the flooding of the River Seine that year.
Shy projectionist Emile has a passion for film and is in love with his coworker at the cinema, Maud, but has trouble admitting his true feelings. His friend, an exuberant inventor and delivery driver, Raoul, picks him up from work to transport him in his bizarre vehicle (called "Catherine"), to pick up a new belt for his projector, which was broken due to Emile's inattentiveness while daydreaming. In the process of purchasing a new belt, Emile also gets himself a new camera (which almost gets stolen by a thief). The story also introduces Lucille, Raoul's childhood friend, who is a cabaret singer at the club L'Oiseau Rare ("The Rare Bird"). Though she is a successful singer, her aunt Carlotta does everything to push her into the arms of the wealthy Police commissioner, Victor Maynott, a man consumed by pride and ambition.

An investigation is made into the whereabouts of the creature by Maynott's second in command, Paté, but is fronted by Maynott, who uses it as a scheme to maintain fear in Paris and increase his chance of being elected in the upcoming mayoral elections. 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 (due to his terrible singing). Whilst trying to vacate the cabaret, Albert stumbles across the creature and tries to get back into the cabaret with no success and flees terrified. Lucille opens the door and accidentally pushes the creature aside into the alleyway after finding it curiously playing with the doorbell. Upon seeing the creature, she flees in an initial state of panic, but later hears the creature sing ("A Monster in Paris") and discovers it is not dangerous but has a lovely singing voice. She welcomes it into her dressing room, and gives it the name, Francœur (meaning "honest heart"), the name of the street where she had found it. The creature is nothing but a flea that the fertilizer, amidst all the other chemicals in laboratory explosion, caused to grow to human scale.

The following day, Maynott opens the MontmartreFunicular, which serves Montmartre and the Basilica of the Sacré Cœur. The trio, along with Francœur and Charles decide to use this opportunity as a way of staging the death of the creature. Things do not go according to plan as Maynott, his pride now turned to corruption, discovers the creature hiding under the trap door of the stage. Trying to escape, Francœur and friends are chased through the streets of Paris by the insane Maynott, who is hellbent on the death of Francœur, even if it means killing Francœur's friends in the process. A chase ensues, involving an airship and "Catherine". The chase concludes at the tip of the Eiffel tower. Meanwhile, Maud, who Emile finally had the courage to make a date with, also arrives there. After a battle to protect Francœur from Maynott, the fertilizer wears off, but a gunshot from Maynott and Francœur's disappearance leads everyone to believe he is dead. Despite believing that he has finally rid himself of Francœur, the megalomaniacal Maynott is then arrested by Paté on the basis that Francœur is innocent. Emile and Maud fall into each other's arms.

In a post-credits scene, Raoul, Lucille, Francœur, Maud, Emile, Charles, Carlotta and Paté (riding the airship from earlier) scatter super-fertilized sunflower seeds to help drain the flooded Seine. In a second scene, Maynott is shown in the same cell as Albert and the camera thief from the movie's beginning, where he is forced to endure their appalling singing.
