After time traveling to the past with Shinobu Oshino, Koyomi Araragi meets Mayoi Hachikuji and returns her backpack which she left in Koyomi’s room. That is when they witness something unidentifiable - something that can only be expressed as the “darkness.”
Koyomi, instinctively feeling danger, sits Mayoi on his bicycle and attempts to escape, but the ever elusive “darkness” tries to swallow the two of them. They barely manage to escape with the help of the young shikigami girl, Yotsugi Ononoki, and they evacuate into the ruins of the cram school. There, Shinobu tells him that she encountered the “darkness” about 400 years ago. Then a woman named Izuko Gaen, who says she knows Meme Oshino, explains to them the nature of the “darkness.” And Koyomi has to face Mayoi’s tragic fate.