Plot

October 24th. Suruga introduces a transfer student Ougi Oshino to Koyomi. According to a self-introduction, she is Meme's niece and likes solving a mystery. Ougi asks Koyomi for help to solve a mystery which she found in audio-visual room at their high school. However, Koyomi and Ougi are trapped in a mystical space as soon as they enter the room. And then, Koyomi has to face his bad memory from 2 years ago.