The narrative is set in Japan. It is the year 2030, and Earth has been fully consumed by a viral illness called "Gibia," which transforms infected individuals into various monsters depending on their age, sex, and race. A samurai and a ninja from the early Edo period journey through time together, arriving in a devastated Japan to assist a professor who is developing a cure for the virus. As a team, they battle numerous Gibia monsters, outlaws, and other dangerous adversaries on their quest to save humanity.