Every summer, 10-year-old Dune goes with her parents to the south-west coast of France, where she meets her best friend Mathilde. Together, they explore pine forests, contemplate the shape of clouds, follow teenagers and crash their parties, and secretly watch horror films. But this one summer is special for Dune – last year, the family skipped their usual holiday without her knowing why. She feels that something has changed. As childhood becomes a little more distant, she begins to see adults and teenagers differently and understand their secrets.

As she struggles to cope with the loss of her older sister, Essie finds herself alone with her father. In the lake that borders the family home, she discovers a mysterious mermaid, which feeds only on gasoline.