Rimo, alternatively known as Li Mo, lives in Shanghai. His childhood unfolds within a late-1990s shikumen neighborhood, where he forges close bonds with Xiao Yu and another friend. After an injury prevents Xiao Yu from attending school, Rimo starts recording lessons for her on cassette tapes. This exchange sparks a deepening connection between them, with romantic feelings blossoming through the recorded messages.

Xiao Yu reveals her family's insistence on applying to a distant elite high school, causing Rimo distress. Determined to stay with her, he commits to intense academic improvement and secures admission to the same school. However, when he shares this success, he discovers Xiao Yu failed the entrance exam and was hospitalized due to parental discipline. Rimo relocates with his family before he can see her, resulting in their separation.

As an adult, Rimo builds a career in architecture. While preparing to move from his apartment, he rediscovers an unlistened-to cassette tape from Xiao Yu. He seeks out an old tape player at his grandmother's former home and hears Xiao Yu's confession: she intentionally failed the exam to remain near him. This revelation brings intense regret over their estrangement.

Driven by this discovery and their shared childhood dream, Rimo renovates a shikumen building into a hostel, crafting a communal space echoing their youthful aspirations. The narrative concludes as Xiao Yu appears at this new establishment, suggesting possible reconciliation or closure. A post-credits scene depicts characters from the film's segments meeting in an airport, with Rimo's direct involvement unspecified.