Xiao Yu spent her childhood in a traditional Shanghai shikumen during the late 1990s, forming a close bond with neighbor Li Mo. After she injured her ankle on a cracked path near their homes, Li Mo carried her back and helped her recover. Unable to attend school, Xiao Yu asked him to record lessons on cassette tapes. This evolved into an exchange of tapes sharing personal thoughts, deepening their connection into romance.

Facing intense family pressure to enter an elite Yangpu District high school, Xiao Yu deliberately failed the entrance exam to stay in Shanghai with Li Mo. Her father violently assaulted her upon learning of her failure, resulting in her hospitalization. This coincided with Li Mo's acceptance into the elite school and his family's move closer to it. Xiao Yu's hospitalization prevented any goodbye.

Years later, Xiao Yu studied abroad while Li Mo studied architecture in Shanghai. During a move, Li Mo discovered an unlistened-to cassette tape from Xiao Yu, revealing her deliberate exam failure and sacrifice. This prompted his unsuccessful search for her.

Eventually, Xiao Yu returned to Shanghai and visited a renovated shikumen guesthouse run by Li Mo and their childhood friend Pan. She appeared at the entrance, sharing a silent, meaningful smile with Li Mo. Later, she reappears among the central characters at an airport.