High school senior Koyomi Araragi discovers a shocking secret one day after helping his classmate Hitagi Senjougahara—a secret he is never to tell.

On his way to Hitagi's house, Koyomi bumps in to Suruga Kanbaru, the school's basketball star and an old friend of Hitagi, of whom he can't get rid of. Later that day, Koyomi is attacked by a mysterious being.