Nagasawa was just an average student... until one night he's bitten by a large, mutated rat on the way home. He's quarantined in the hospital for a while, but when he's released, all seems normal. But soon he's drawn to a man, that bears similar mutations to the rat, and Nagasawa kills him with ease. It turns out that Nagasawa is infected with a disease called "Nightshift" and he's a special case that let's him fight the insane infected, and he's employed my a medical agency to do just that.