"I am the companion of someone who wants me to be his companion. That's all!" Kuroi Souya, a high school student who has lost his past memories and lives a peaceful life. One day, his city is suddenly attacked by a mysterious giant weapon called the "Nebula Weapon". Souya is involved in the battle with his cat-like "teacher" and Gothic Lolita Ginko, but - he didn't expect that his opponents are the seven heroes who are protecting the city! What is the reason for the battle hidden in Souya's memory?