Hatsuki, a high‑school student, lives with her older sister Hatsumi, whom she has a huge crush on. On Hatsumi’s 16th birthday, she is suddenly surrounded by a green light and disappears right in front of Hatsuki. With the help of a creature that literally resembles a plump baby chick, Hatsuki follows her sister into a place called the Great Library, a vast archive that contains countless worlds bound inside books. Though Hatsumi is gone, Hatsuki begins a quest to find her beloved, traveling from book to book in search of her sister.