A peculiar transfer student named Ougi Oshino has just arrived at Naoetsu Private High School. She is quickly introduced to senior student Koyomi Araragi by their mutual friend Kanbaru Suruga, in hopes of obtaining advice regarding a strange discovery she has made. After taking a look at the school's layout, Ougi notices that a classroom has appeared in an otherwise empty area—a place that should not exist.
Unsure if this is the work of an apparition, Araragi and Ougi attempt to unravel the truth behind this enigma. But Araragi soon discovers, after finding himself locked in with Ougi, that the room holds the memory of an event he had long since forgotten.
Summary taken from: MyAnimeList