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 is a high school student who lives with her sister Hatsumi and harbors a deep crush on her. On Hatsumi's sixteenth birthday, a green light engulfs her and she vanishes before Hatsuki's eyes. Aided by a creature that looks like a plump baby chick, Hatsuki follows her into the Great Library, a place where different worlds are kept inside books. Hatsumi is nowhere to be found there, so Hatsuki sets out to search for her by traveling from one book world to the next. The show mixes fantasy adventure and mystery with an ecchi streak.