It's the end of the 21st century, and a strange plague called "Cagaster" has appeared. One-in-a-thousand people are infected—turning humans into monstrous cannibalistic insects. Two-thirds of humanity is decimated. 30 years later, young expert bug exterminator and mercenary, Kidou, and newfound friend, Ilie, struggle to survive in this brutal new world, all while delving into the mysteries of the plague and its origins.