Julien, an aging widower, is a passionate butterfly collector. Elsa, an eight-year-old girl, with her mother, just moved into his apartment building. The mother is usually away, and Elsa starts visiting Julien. One day, Julien decides to go to the Vercors plateau in search for a rare butterfly called Isabelle which can live for only 72 hours. Elsa decides to join his adventure, without telling him.
The story complicates when Elsa's mother reports her daughter kidnapped.