500 years ago, a sword demon and a dragon mated, giving birth to a "half-man, half-dragon" that defies the laws of the world. In rainy Tokyo, a young boy named Onishi Shinji is picked up by a dojo and lives a sleepy life as a "normal high school student." But when the "darkness" that targets Aya makes its move, the sleeping beast awakens. "Shin-gan" is a secret technique that cuts through the "core" rather than the appearance. In the modern world, disguised as everyday life, the battle between dragon and sword begins again.