Spoiled rich since birth, bored playboy Kiito Sonomiya starts a new year at school in the hopes that a girl of some interest will come his way. His wish comes true when a black-haired beauty confronts him—only to completely pummel him! Left indignant and full of vengeance, Kiito vows to find the mysterious girl and take revenge. His burning desire leaves him no time to pay attention to the popular new student, the angelic Yui Kamio, notable for her kind personality, stunning looks, and white, chain-bound hair.

One evening, Kiito runs into Yui and discovers she holds a secret. When the chain binding her hair is taken off, she transforms: both her clothes and hair turn black, and her personality takes a violent 180 degree turn. Kiito soon realizes that the black-haired girl he encountered earlier and Yui are one and the same.

Quickly tying the chain back and returning her to normal, Yui's friend Nao Ebisuno explains that it is all that restrains Yui's violent side. Now aware of her secret, Nao enlists Kiito to protect "Yui in White" and keep "Yui in Black" from coming forth and causing trouble. Finding himself drawn to this endeavor, Kiito is now dragged out of his comfortable life and into the wild world of Yui Kamio.