Based on the erotic game with the same name by Root. This anime can be quite confusing if you haven't played the game.
Hatsuki is a high school student living alone with her classmate, Hatsumi, on whom she has a huge crush. Tragedy strikes on Hatsumi's 16th birthday, when she is suddenly surrounded by green light and disappears. Hatsuki manages to follow Hatsumi with the help of a being resembling a fat chick (bird), and ends up in a place called the Great Library — a hub to many different worlds that can be visited via the books there. However, Hatsumi is nowhere to be found, so Hatsuki sets off on a journey from book to book searching for her lost love.