Shen Yan, an old-book restorer, discovers a copper button marked with the number seven and an altered library card inside a bookshop on Fog Lamp Alley, uncovering suspicions surrounding the theft of the community's blue box. Xu Tang, a clock repairer who once repaired the blue box's lock, becomes a suspect in the eyes of archivist Liu He. The two visit Zhou Bo, the teahouse owner, and compare receipts and engravings, eventually discovering that Gu Chi, the old bookshop owner, secretly switched the blue box and left clues in the form of a button to protect his mother's old letters. Once the truth comes to light, the blue box is returned to its place, Xu Tang is cleared of suspicion, and the understanding between Shen Yan and Xu Tang grows deeper.