Gaspard Snow, Nicolas Duvauchelle, is a 40-year-old crooner performing at the Flowerburger, a bohemian-style cabaret housed on a barge that his grandmother founded in the early 1940s and which his aging father Camille, Tchéky Karyo, now runs. With the venue perilously close to bankruptcy, Camille contemplates selling the place, but Gaspard refuses to part with his grandmother’s legacy, still haunted by the deaths of both his mother and his grandmother and unsure how to move forward.
One night, while walking home along the Seine, Gaspard discovers an unconscious mermaid washed ashore. He rushes her to a hospital, where a doctor named Victor—who is utterly entranced by her singing—cannot help but be consumed by the moment and dies suddenly. The hospital’s doors close to admitting her, so Gaspard carries the mermaid to his apartment and places her in his bathtub. When Lula emerges from her slumber, she attempts to hypnotize him with her song, but Gaspard remains immune. As he tends to a tail wound, she introduces herself as Lula, and she explains that she must return to the ocean within two days or she will die; still, Gaspard begs her to stay long enough for the wound to heal.
The following morning, Lula learns that Gaspard is still alive and that most men who hear her sing fall in love to the point of heartbreak. Gaspard, who has lost faith in love after heartbreaks of his own, is surprised to find himself drawn to the enigmatic visitor. Before long, Lula’s arrival is noticed by the block’s lively neighbor Rossy, Rossy de Palma, a nosy and eccentric presence who mistakes Lula for Gaspard’s new girlfriend and ends up sharing the tale of the Flowerburger’s fragile fate.
Meanwhile, Milena, Victor’s pregnant girlfriend and a doctor herself, is devastated by his sudden demise and begins to investigate the mystery surrounding his death. She discovers a pop-up book left by Gaspard at the hospital, a chronicle of the Flowerburger’s history that his grandmother had given him. A strange blue, gooey substance found outside the hospital is identified by a biologist as the blood of an unknown creature that cannot survive out of the ocean. Hearing rumors of a mermaid sighting in Paris, Milena becomes determined to find Lula and uncover the truth behind Victor’s death.
Back in Gaspard’s bathtub, Lula accidentally starts a small fire after smoking a cigarette left by Rossy, dropping the cigarette and nearly burning the building. Gaspard rushes back to rescue Lula and moves her to Rossy’s bathtub. Despite the chaos, the two grow closer, and Gaspard proposes a farewell party at the Flowerburger before Lula must return to the sea.
That night at the Flowerburger, Lula is introduced to Camille, who is delighted to see his son seemingly in love again. Lula confesses that her mother was killed by a sailor and that she has since killed several sailors herself in revenge. Milena arrives at the Flowerburger searching for Lula, and Gaspard escorts Lula to a shark tank, where he dives in to swim with her and they share a kiss beneath the water.
The next morning, Gaspard hesitates to let Lula go. He leaves to fetch her a fish, while Lula pleads with Rossy to help her escape, fearing for Gaspard’s safety. When Gaspard returns, Milena and the biologist have captured Lula’s blood samples, and Milena intends to stop Lula from claiming more victims. Demanding to know Lula’s whereabouts, Gaspard faints and is taken to the hospital, where Milena keeps Lula under guard and continues extracting her blood.
With Rossy disguising herself as a nurse to create a distraction, Gaspard manages to lead Lula away from Paris, with Milena in hot pursuit. They flee toward a river, where Gaspard plunges Lula into the water to keep her safe, and she swims away. As Gaspard faints, Milena experiences a vision in which she mistakes him for Victor and resumes his heart with CPR. Returning home, Gaspard receives a bottle containing Lula’s priceless tears of pearls from the biologist, a token that enables him to buy the Flowerburger from his father. He then sets sail with the vessel, hopeful that one day he and Lula will be reunited.
