Hatsuki's obsession with her sister isn't just the hook, it's the entire show, and the library is just a maze the narrative puts you through to understand what that obsession actually means. The middle gets messy and scattered, but that confusion mirrors how love scrambles your sense of reality. By the end, the show rewires what you thought you were watching, and you realize the journey through those worlds was always about one girl trying to understand the difference between desire and devotion.
Hatsuki's perfectly ordinary life shatters when her sister Hatsumi vanishes in a burst of green light on her sixteenth birthday. Desperate to find her, Hatsuki chases the only clue she has: a peculiar creature that resembles nothing so much as a chubby, floating chick. Together they tumble into the Great Library, a vast repository where entire worlds exist trapped inside books. But Hatsumi isn't here. Instead, Hatsuki discovers she must journey through countless literary worlds, each one stranger and more dangerous than the last, searching for any trace of her missing sister. Along the way she encounters ridiculous characters, genuine threats, and the constant mystery of what force orchestrated Hatsumi's disappearance in the first place. This is adventure that doesn't take itself entirely seriously, blending fantasy exploration with genuine emotional stakes. The show moves quickly from world to world, refusing to stay too long in any one place. If you want something that mixes mystery, whimsy, and just enough weirdness to keep you guessing, this scratches that itch.