Adèle, a 15-year-old high school student with an introverted nature, experiences a transformative encounter when she spots an older woman with striking blue hair on the street, sparking an instant attraction. Although she initially loses her virginity to Thomas, a classmate, she feels unfulfilled and decides to end that relationship. This leads Adèle to grapple with her sexual identity, especially after she has intense fantasies about the blue-haired woman while masturbating and shares a kiss with her friend, Béatrice, who ultimately rejects any further advances and advises Adèle to forget the moment.
Seeking clarity, Adèle’s openly gay friend, Valentin, takes her to a lively gay dance bar. Unexpectedly, she wanders into a lesbian bar where she meets Emma, the blue-haired woman, who claims Adèle is her cousin to fend off unwanted attention. This marks the beginning of a deep friendship between Emma and Adèle. When Emma starts attending Adèle’s school, her friends begin to suspect her sexual orientation, leading to social ostracism.
As their bond strengthens, Adèle and Emma share a kiss during a picnic, which evolves into a passionate sexual relationship. Adèle finds herself accepted by Emma's artistic family, while she must maintain the facade to her conservative, working-class parents that Emma is merely her philosophy tutor. On her surprise 18th birthday, Adèle celebrates with old friends, but Emma's absence is notable and palpable.
As the years unfold, they take the step to live together. Adèle secures a job as an elementary school teacher, while Emma focuses on her painting career, organizing frequent house parties. During a particularly lively gathering, Adèle meets Emma’s pregnant colleague Lise, gallery owner Joachim, and aspiring actor Samir. While conversations about America and relationships swirl around the room, Adèle forms a friendship with Samir, but Emma spends her time with Lise, intensifying Adèle's feelings of loneliness.
The relationship's strain becomes evident as Emma belittles Adèle’s career choices while pushing her to pursue writing. Despite her insistence that she's content as she is, tensions rise between them. After a night of working late, Emma returns home to find Adèle has betrayed her trust by sleeping with a male colleague. In a fit of rage, Emma decides to end their relationship.
Three years pass, and fate brings them together at a restaurant. Adèle, happy with her role as a kindergarten teacher, is still secretly longing for Emma. Meanwhile, Emma is entrenched in a stable relationship with Lise, assisting in raising Lise’s three-year-old child. Even though Emma expresses that she has no sexual satisfaction in her current relationship, she admits to Adèle that she no longer loves her. Nonetheless, she acknowledges the profound connection they once shared, promising to always carry "infinite tenderness" for her. With apologies and tears, they part ways.
The narrative reaches a poignant moment when Adèle visits Emma’s art exhibition. Among the displayed artworks hangs a nude portrait of Adèle, which Emma created during their time together. While Emma acknowledges Adèle’s presence, her focus lies on Lise and the other attendees. Adèle congratulates Emma on her success, and after a fleeting conversation with Samir, she departs, walking away into the distance as Samir rushes after her but mistakenly goes the wrong way.
