High school student Mihara Ann spends her summer vacation on a homestay at a private library on the outskirts of Sapporo, commonly known as the "Library Manor." Upon breaking the rules set by its unsociable owner, Sage, Ann accidentally wanders into the library at midnight. There, she finds herself in a decayed, alternate world—the "Library Labyrinth," where phantom remnants of forgotten stories roam. Tasked with becoming the labyrinth's librarian, Ann is ordered to "restore the labyrinth by encouraging more people to read the library's books." In an old mansion surrounded by beautiful nature, the story of a timid girl begins to take color, bringing new life to her world and the forgotten tales.