Saitama is an anonymous office worker who resides in Z City, a fictional metropolis in Japan. After being fired, he decides to face the monsters and villains that plague the world. After a hard training that ends up causing him to go completely bald, he manages to become the most powerful hero on the planet, capable of finishing off any enemy with a single punch.

U.C. 0097, one year after the opening of "Laplace's Box". Despite the revelation of the Universal Century Charter that acknowledges the existence and rights of Newtypes, the framework of the world has not been greatly altered. The conflict later dubbed the "Laplace Incident" is thought to have ended with the downfall of the Neo Zeon remnants known as the Sleeves. In its final battle, two full psycho-frame mobile suits displayed power beyond human understanding. The white unicorn and the black lion were sealed away to remove this danger from people's consciousness, and they should now be completely forgotten. However, the RX-0 Unicorn Gundam 03, which disappeared two years earlier, is now about to show itself in the Earth Sphere once more. A golden phoenix… named Phenex.

After a mysterious virus wipes out ninety percent of humanity, vampires emerge from the recesses of the earth to enslave mankind, treating them like livestock. After growing tired of donating blood in exchange for protection, 12-year-old Yuichiro Hyakuya and his best friend Mikaela Hyakuya plot an escape along with the other orphans. After being the only successful runaway, Yuichiro joins the Moon Demon Company, an extermination unit of the Japanese Imperial Demon Army dedicated to killing off vampires.