It’s summer in a country house in the south of France. A family decides to organize a big reunion there in July. There is Bernard, a conservative and somewhat rigid doctor, and his sister Lise, his opposite, a modern and libertine architect, as well as the teenagers Patrick and his brother Julien who are both attracted to Dina, a 15-year-old red-headed girl with green eyes. She is more than her age and is aware of the power of her charms. “She’s my cousin, and my first great love,” says Patrick from the beginning of the film.
Patrick writes down all his wishes and dreams in a journal, and his thoughts often return to Dina. She flirts freely with the two boys, but is only waiting for one thing: for Patrick, a shy and reserved boy, to come to her. Julien, on the other hand, is more reckless, and never hesitates to approach Dina, so much so that he seems vain.
When Julien discovers Patrick’s diary, he fetches Dina and reads it in front of her, mocking her. Dina takes the diary, reads Patrick’s poems and realizes that Patrick is in love with her. In a moment of distraction, Julian throws himself on her and touches her breasts. Dina slaps him. Julien leaves, offended. Dina reads Patrick’s diary and indulges in erotic fantasies.
On the way home from the nearby train station, Lise, the two boys’ aunt, discusses sex with Patrick. She understands that he is a virgin and takes him out to the field to introduce him to sexuality. She wants to teach him how to behave with Dina. Patrick and Lise have sex, and he notes this “big event” in his journal. When Dina reads it, she is very shocked. Lise arrives and reacts with wit. She explains to him that these are just Patrick’s fantasies and that nothing has happened between them and manages to reassure Dina.
Dina turns 16. She dances with Patrick and they kiss for the first time. Meanwhile, an argument breaks out between Bernard and Julien who runs away angry. After the party, Patrick isolates himself with Dina who, in light clothing, was waiting only for him. He undresses her, strips her naked on a bed, begins to caress her everywhere and then they make love.
The same night, the phone rings. It’s Julien calling. Lise goes to pick him up. On the way, Julien explains to her his disagreement with her father. They spend the night together.
The next morning, Dina and Patrick, as well as Lise and Julien wake up in each other’s arms. A new summer day begins.