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 Parasytes 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 travelling 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.

In an attempt to colonize Mars, 21st century scientists were tasked with warming up the planet so that humans could survive on its surface. They came up with an efficient and cost effective plan of sending cockroaches and mold to the surface so that the mold would absorb the sunlight and purify the atmosphere, while feeding the insects whose corpses would help the mold spread throughout the planet. Five hundred years later, the first manned ship to Mars lands on the planet and the six crew members are attacked by giant mutated humanoid cockroaches with incredible physical strength later labeled as Terraformars. Before the crew members are easily wiped out, they manage only to send a transmission back to Earth. Decades later, a multinational expedition is sent to exterminate the mutated bugs and take control of the planet just to be wiped out as well, except for two survivors who managed to return to Earth, with one of them swearing to one day return to Mars and avenge their fallen companions. As a third expedition is assembled, questions are raised about the true origin of the Terraformars and their connection with a mysterious disease that is afflicting mankind back on Earth.To fight the Terraformars' strength and agility, the members of the second and third expedition undergo genetic modification to inherit the characteristics of other organisms, only possible after having a special organ implanted inside their bodies with only a 30% chance of surviving the surgery.

In the late 26th century, overpopulation on Earth is reaching the breaking point, and humanity must find new frontiers. The terraforming of Mars has taken centuries but is now complete. The colonization of Mars by humanity is an epoch-making event, but an unintended side effect of the terraforming process unleashes a horror no one could ever have imagined... After humanity's first manned mission to the Red Planet was lost, a second expedition arrives. The explorers prepare to exterminate the cockroaches that were used to transform the Martian environment. They are shocked to discover that the insects have mutated into giant, aggressive humanoids with one overriding goal — exterminate the humans! But this crew of explorers has each undergone the Bugs Procedure, terrifying experimental surgery designed to make them more than human...

The protagonist, Chihiro Furuya, is a male highschool student with a keen interest in zombies, collecting zombie-related videogames, film and manga, and even to the point of desiring to “kiss a zombie girl”. Following the death of his pet cat, he attempts to revive it using an old manuscript, which describes the process of creating a potion for resurrection. At this time he encounters a girl named Rea Sanka, who has run away from home. In an attempt to commit suicide, she drinks a sample of the “resurrection” potion which is created from a poisonous herb known as Hydrangea macrophylla, however this fails to kill her. Following an argument with her father, she falls from a cliff by accident and dies; however as a result of the potion, she returns as a zombie. Though she becomes undead, she chooses not to consume human flesh, and rather survives on eating the hydrangea herbs.