The blue-haired girl comes to talk to Clémentine and introduces herself as Emma. Clémentine sees the blue-haired young woman again with Sabine at a lesbian bar. One evening shortly thereafter, Valentin takes Clémentine to some gay bars. Feeling depressed, she is helped by one of her male friends, Valentin, to whom she confesses everything Valentin tells her that he has already dated a boy, which Clémentine finds quite comforting. Six months later, however, Clémentine is unable to have sex with Thomas and breaks up with him.

Unable to forget this encounter she starts to have doubts about her sexuality-but decides to date Thomas because she wants to feel normal. For Clémentine, it is love at first sight. In the beginning, Clémentine meets a boy, Thomas, who is a student in Terminale (final year of lycée, the French equivalent of senior high school or sixth-form college) they like each other, but soon afterwards, Clémentine becomes intrigued by a chance meeting with a blue-haired young woman on the arm of another woman named Sabine Decocq. The story then follows Emma as she reads Clémentine's diary, which tells the whole story of the relationship between the two young women from Clémentine's teenage years and her first meeting with Emma to her untimely death. Emma must face the hostility of Clémentine's father-somewhat off-set by Clémentine's welcoming mother. After the death of her partner Clémentine, Emma goes to the home of Clémentine's parents, Daniel and Fabienne, in accordance with Clémentine's will, to request access to Clémentine's personal diary.

The story takes place in France between the years of 19.

Emma blue is the warmest colour