In the near future, Tokyo was left flattened as a result from a great earthquake. A new city, MegaTokyo, was then recreated due in no small part from the aid of a multi-million dollar company, Genom Corp. Genom created and mass-produced biomechanical creatures called Boomers to aid in the restoration of MegaTokyo. When the Boomers began to run out of control, the ADPolice at first tried to stop them, but they proved to be far more difficult to deal with than was first imagined. Under the ever looming Boomer threat, a group of four girls from varying degrees of society banded together. Calling themselves The Knight Sabers, they were the only ones with enough firepower and resourcefullness to defend the fledgling MegaTokyo from Genom and its berserk Boomers.
Based on Iwaaki Hitoshi's science-fiction horror manga by the same name.
The 17-year-old Izumi Shinichi lives with his mother and father in a quiet neighborhood in Tokyo. One night, worm-like aliens called Parasytes invade Earth, taking over the brains of human hosts by entering through their ears or noses. One Parasyte attempts to crawl into Shinichi's ear while he sleeps, but fails since he is wearing headphones, and enters his body by burrowing into his arm instead, taking over his right hand and is named Migi.
Because Shinichi was able to prevent Migi from traveling further up into his brain, both beings retain their separate intellect and personality. As the duo encounter other Parasytes, they capitalize on their strange situation and gradually form a strong bond, working together to survive. This gives them an edge in battling other Parasytes, who frequently attack the pair upon realization that Shinichi's human brain is still intact. Shinichi feels compelled to fight other Parasytes, who devour humans as food, while enlisting Migi's help.