When a frail little girl suddenly develops a passion for inventing, her family and friends are left baffled. "What’s gotten into Myne?" they ask, unaware that the answer lies not in a “What,” but in a “Who.” That “Who” is Urano Motosu, a book-loving apprentice librarian who perished in a Tokyo earthquake and has inexplicably found herself inhabiting Myne’s body. Since Myne’s world is still in a medieval era where books are reserved for the wealthy, the new Myne is determined to share her cherished books with everyone during her limited time.