While Kou is asleep in his bed, the scream of a girl wakes him up. He rises to see in front of him a battle full of gunshots and flashing bombs exploding. He realizes that he is wearing an iron armor. "What's this?" He doesn't know what's going on.
He leaves the virtual world with Rain, who says she is his junior partner. They arrive in a ruined city, and Kou learns that he is a special Simulacrum user who graduated from school several years ago. He has forgotten his memories that may have had a significant impact on his life. The name of the case that slowly materializes from his lost memory, "Gray Christmas."
"What is Gray Christmas? Why was I investigating the case?" He comes to remember the whole story of the case and its meaning. A peaceful life that was meant to last forever, which however, suddenly comes to an end...
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...
