In September 1963, the story follows the experiences of Anne Weber, a shy and introverted 13-year-old girl, and her slightly older sister Frédérique, during a summer spent with their recently divorced father on the coast. After their holiday, they return to Paris to live with their mother, where they face the challenges of adolescence in a strict all-girls school, the Lycée Jules-Ferry.
At school, Anne often feels out of place. She struggles with her academic performance and is occasionally ridiculed: a teacher mocks her for a poorly drawn picture during art class. She is also preoccupied with superficial worries, such as her mother not buying her stockings—which are considered essential at her school—and anxiously anticipates her first period. She perceives menstruation as a milestone marking womanhood, leading her to feign menstrual cramps even though she hasn't started her period yet. Social conversations with her friends revolve around their limited understanding of sex, which they find both humorous and confusing. Anne also secretly reads the letters exchanged between her sister Frédérique and her boyfriend Marc, and falsely claims to her friends that he is her own boyfriend, fueling her longing for romantic connection.
Meanwhile, Frédérique begins to shift her interests away from her relationship with Marc. She becomes increasingly involved in political activism, especially concerning the Algerian War and her Jewish identity. Her newfound political awareness causes tension with her wealthy and privileged friend, Perrine. Frédérique develops a close friendship with the outspoken Pascale, who shares alarming stories with the class about a peaceful protest against far-right extremists that was violently suppressed by the police, exposing Frédérique to the harsh realities of political struggle.
The social fabric among the friends becomes more complex when rumors spread about Muriel, another student who runs away from home. Her father, visiting her, makes an inappropriate advance just before she leaves. Muriel eventually returns home briefly, revealing she has dropped out of school and now lives on a farm with her boyfriend. During this turbulent period, Frédérique also becomes involved in a school play, which marks a significant personal achievement. However, on the opening night, her father attends but disappointingly leaves without staying to praise her performance, leaving her feeling unrecognized.
Throughout this time, Frédérique and Anne return to their father's seaside home for the summer, where new dynamics and feelings continue to unfold as they confront growing up, identity, and their familial relationships. The narrative captures the innocence and confusion of youth against the backdrop of a society on the brink of change, illustrating the personal struggles intertwined with larger political and social issues of the early 1960s in France.
