Ougi Oshino, a strange transfer student, has just arrived at Naoetsu Private High School. Kanbaru Suruga, a common acquaintance, promptly introduces her to senior student Koyomi Araragi in the hopes of getting assistance on a bizarre discovery she has discovered. Ougi examines the school's plan and discovers that a classroom has arisen in an otherwise empty area—a location that should not exist. Unsure if this is the work of an apparition, Araragi and Ougi strive to solve the mystery. However, Araragi soon realizes, after being confined in alongside Ougi, that the chamber contains the memories of an incident he had long forgotten.