In 2071, roughly fifty years after an accident with a hyperspace gateway made the Earth almost uninhabitable, humanity has colonized most of the rocky planets and moons of the Solar System. Amid a rising crime rate, the Inter Solar System Police (ISSP) set up a legalized contract system, in which registered bounty hunters, also referred to as "Cowboys", chase criminals and bring them in alive in return for a reward.

Bayron City -- a Mega-Float type city in the Pacific Ocean, which doesn't belong to any particular nation. As the mine for Orgonium, a new energy resource, this city is the most noticed in the world right now, and here special cases caused by demons called "D Hazards" has been occurring frequently. Only a handful of people know about the existence of D Hazards. They are handled by PMCs (Private Military Companies). Shu is a young man living in Bayron City who runs such a company, but his company is tiny. On top of that, he cherry-picks his jobs, so he is always in a bind for money. His life, both publicly and privately, is being supported by the beautiful high school girl Kisara who attends a school in Bayron City.

Suzuki's a high school girl in love, but the guy she's fallen for is nothing like her! While she's cheerful, outgoing, and always trying to fit in, her classmate Yusuke Tani is stoic, quiet, and doesn't seem to care what people think of him. Will Suzuki be able to overcome her anxieties and ask him out, or will she discover that opposites really don't attract?

Yuki Takeya, Kurumi Ebisuzawa, Yuri Wakasa, and Miki Naoki decide to stay over at school. Along with the school adviser Megumi Sakura, they suddenly find themselves to be the final survivors of a zombie attack, and continue to live and survive at the school.