One day, Kadokura Kou awakes in a virtual battlefield. He, who is a skilled Simulacrum user, soon remembers that he was pursuing someone with Rain, his partner. Kou and Rain investigate the incident that ruined his peaceful school life known as "Gray Christmas", but various mysteries stand in the way of the truth. As time passes, he gradually regains his lost memories and re-unites with his friends. When all of the mysteries congregate, the door of truth opens...