Hidden away on a remote mountaintop, the Kokoro Library struggles to attract many visitors. Nevertheless, it is the home of sisters Iina, Aruto, and Kokoro, who consider it their mission to keep the library running in their parents' stead. Kokoro has long dreamed of becoming a librarian at the library with the same name as her, and tries her best to keep up with her older sisters.

Due to the lack of patrons, the girls mainly spend their time sipping tea, chatting, and reading. However, as the number of mysterious visitors progressively increases, Kokoro begins to learn about the strange history of Kokoro Library—a place where small miracles seem to blossom.