One day, an unknown sapphire spherical life form suddenly appeared in the world, causing harm to humans. The medium and large spheres can hurt people, and the small spheres can attack people. In the crisis, high school students across the country receive military training and are organized into a reserve army of soldiers. They begin to fight with guns in various regions across the country. The task of the reserve soldiers is to defend and destroy the small spheres deployed on the front line moving toward the city center. It is mainly a story about high school students becoming soldiers and starting to fight with guns.