Wing Hedwig Tam and her best friend Sylvia are students at a 2001 all-girls Catholic school, navigating the thrill and confusion of young love under strict social rules. Wing secretly loves Mr. Wong Stephen Au Kam-Tong, the arts teacher, and writes him a confession letter. When Sylvia, without asking Wing, passes the letter to Wong, his response is firm: he rejects the romance and tells Wing to be mindful of her feelings. Humiliation follows as Wing confronts Sylvia, who reveals she is actually in love with Wing. The next day, Sylvia tries to insist it was a joke, leaving Wing to doubt her. The pair decide not to dwell on it for the moment and continue as best friends for the semester.
Encouraged by a shared dream, Sylvia invites Wing to help make a short film about their lives. While filming, they recreate a scene inspired by Days of Being Wild, and in that moment they share their first kiss. Their bond deepens, but a school social worker discovers their kiss, triggering formal reports to the principal. The school enlists their parents for a meeting. Wing's father offers quiet support, while Sylvia's father speaks of financial strain and urges his daughter to maintain her scholarship and behave to protect it.
Back at school, Wing buys a ring for Sylvia and plans a future together, but Sylvia responds coolly and hints at a breakup, suggesting their feelings are only puppy love. Unwilling to accept this, Wing chases her to the playground and kisses her in front of others; teachers intervene, scolding them and forcing apologies. During the apologies, Sylvia tears hers up, assumes responsibility, and agrees that she initiated the kiss, ultimately quitting school. Wing searches for Sylvia, but she vanishes from her life.
By 2003, Wing has moved on to university to study sociology and meets Kin-long Chan Kin-Long, her first male friend, who harbors feelings for her. A chance encounter with Sylvia happens again, and Sylvia reveals she now has a boyfriend and that they have been intimate. Heartbroken, Wing seeks solace in alcohol and, in a moment of revenge, sleeps with Kin-long but finds no real pleasure. Soon after, Sylvia calls Wing, and Wing rushes to the hospital to comfort her as her father dies of heart failure. Sylvia tries to stay strong for her siblings but breaks down alone with Wing, who offers comfort and a sense of reconciliation. Sylvia suggests that if they aren’t married by 30, they should marry each other, but they agree not to see one another until that day.
Wing learns that Sylvia has dropped out of film school to support her siblings. To honor Sylvia's dream, Wing transfers to film school herself and begins working as an assistant director. Inspired by their shared past, she decides to craft a film based on their love story. As the countdown to 30 approaches, Sylvia phones to announce her upcoming marriage and invites Wing to be a bridesmaid. On the wedding day, when the pastor reads the vows, Sylvia looks at Wing and both of them cry; Wing mouths “yes” and Sylvia repeats “yes” to her groom. In the end, Wing returns to her former school and dances alone in the hall, finally letting go of their relationship.
