Mankind's prosperity has led to a vast population increase and serious food shortages. In an attempt to solve the food shortage problem, scientists create a new species to use for food, and place the herd in northern Japan. In time, the creatures mutate and attack the people of Japan. An international peacekeeping organization called the Armed Volunteers tries to stop the creatures, now referred to as "ruffians," but they also oppress the Japanese.
Another group, referring to itself as the Savior Group and led by a mysterious woman named Achi, rises up to defend Japan against the Ruffians and Armed Volunteers.