Xiao Yu grows up in a Shanghai shikumen neighborhood alongside childhood friends Li Mo and Pan. Her family, particularly her father, exerts strong pressure on her to gain admission to the elite Yangpu High School. A significant turning point occurs when she sustains an ankle injury on cracked pavement near their homes. During her recovery, Li Mo begins carrying her regularly, and they initiate communication through exchanged cassette tapes. This exchange deepens into a mutual romantic relationship.

Facing separation due to her family's insistence she attend Yangpu High, Xiao Yu makes a pivotal choice: she deliberately fails the entrance examination to stay in Shanghai with Li Mo. This act of sacrifice results in severe physical punishment from her father, necessitating hospitalization. Concurrently, Li Mo gains acceptance to Yangpu High. Their paths diverge when his family relocates near the school; he remains unaware of her deliberate failure and sacrifice. Years later, Li Mo discovers an unheard cassette tape Xiao Yu had given him. Listening reveals her confession about failing the exam to stay with him and her enduring feelings.

Physically, Xiao Yu wears her hair in a ponytail during youth, later adopting a shorter style. Her adolescent height reaches approximately Li Mo's shoulder level during their piggyback rides. Her personality demonstrates quiet thoughtfulness and deep emotional investment. Actions like meticulously recording school lessons for Li Mo while injured and risking parental disapproval to maintain their connection underscore her loyalty and capacity for sacrifice.

In the narrative's conclusion, set around 2008, Xiao Yu returns to Shanghai after studying abroad. She arrives at a guesthouse operated by Li Mo and Pan within a renovated shikumen building. A silent, mutual smile between her and Li Mo implies reconciliation and the potential rekindling of their relationship. A post-credits scene confirms her presence alongside the other protagonists in an airport setting.