During October of his third year in high school, Araragi Koyomi is introduced to a transfer student named Oshino Ougi by his underclassman Kanbaru Suruga. Ougi wants to ask Koyomi’s opinion on a strange discovery of hers. She draws him a map of Naoetsu High School and, judging by it, there seems to be something wrong with the building’s floor plan. This observation as starting point, a tale never meant to be told begins to emerge changing Koyomi’s high school life forever. It is a story that reveals what made Araragi Koyomi who he is now. It is a story that reveals how it all began.
— adapted from Crunchyroll