"Parasyte" is a science fiction horror manga that explores the sudden invasion of Earth by parasitic creatures. These parasites, resembling small worms, infiltrate human hosts by burrowing into their brains and taking control of their bodies. Once in control, they can manipulate the host's form, often transforming parts of the body into deadly weapons. The parasites' ultimate goal is to survive and dominate the human race, using their hosts to hunt and consume other humans.

The protagonist, Shinichi Izumi, is a high school student who becomes the unwilling host of a parasite. However, in his case, the parasite fails to reach his brain and instead takes over his right hand, naming itself Migi. Unlike other parasites, Migi retains its intelligence and forms a symbiotic relationship with Shinichi. The two must coexist, as Migi cannot survive without Shinichi, and Shinichi gains enhanced physical abilities and a powerful ally in his fight against other parasites.

As the story progresses, Shinichi and Migi encounter various other parasites, each with different levels of integration and control over their hosts. Some parasites, like Migi, develop unique personalities and relationships with their hosts, while others remain purely predatory. Shinichi's encounters with these creatures force him to confront moral and ethical dilemmas, as he struggles to protect humanity while understanding the nature of the parasites.

Key characters include Ryoko Tamiya, a parasite who takes over the body of a schoolteacher and becomes a central figure in the parasites' attempts to understand and coexist with humans. Kana Kimishima, a classmate of Shinichi's, develops a psychic connection to the parasites, which leads to tragic consequences. Uragami, a human serial killer, becomes entangled with the parasites, adding another layer of complexity to the narrative.

The manga delves into themes of identity, coexistence, and the blurred lines between humanity and monstrosity. Shinichi's transformation, both physically and emotionally, is a central focus as he grapples with the changes brought about by his symbiotic relationship with Migi. The narrative builds tension through a series of confrontations and battles, culminating in a broader exploration of the parasites' origins and their impact on the world.

"Parasyte" combines intense action with philosophical questions, creating a story that is both thrilling and thought-provoking. The dynamic between Shinichi and Migi serves as the emotional core, driving the plot forward while examining the complexities of their unique bond. The series maintains a balance between horror and introspection, making it a compelling read for fans of the genre.