Strange parasitic creatures fall from the sky and begin taking over human hosts by consuming their brains. These beings can morph their hosts heads into razor-sharp tentacles and blades, and they survive by feeding on other humans. In response, a specialized South Korean task force called Team Grey is formed to track and eliminate the parasites, led by the hardened Choi Jun-kyung, whose own husband is a captive parasite she uses as a hunting dog to sense others of its kind.

The story centers on Jeong Su-in, a quiet supermarket cashier with a traumatic past involving an abusive father. After she is stabbed by a disgruntled customer, a parasite attempts to take over her brain but fails because its energy is diverted to healing her severe wounds. This creates a unique hybrid: Su-in retains her human consciousness while a parasite she later names Heidi lives in her subconscious, able to emerge and control the left side of her head to protect them both. This coexistence is feared by both humans and full parasites.

Su-ins path crosses with Seol Kang-woo, a small-time gangster searching for his missing younger sister. He discovers that his older sister has been taken over by a parasite and that the creatures are gathering at a secluded church led by a charismatic pastor parasite. When Kang-woo finds his sisters body at the church, he and Su-in are forced to flee together. Meanwhile, Senior Detective Kim Chul-min, who rescued Su-in from her father when she was a child, begins investigating the case and grows suspicious of her condition. He teams up with Kang-woo to rescue Su-in after she is captured by Team Grey, but Chul-min is eventually killed by the pastor parasite, who then assumes his form to infiltrate the task force.

The narrative builds toward the parasites plan to take over the mayor of Namcheons body during a public festival. Su-in forms an uneasy alliance with Kang-woos infected older sister, whose parasite feels betrayed by the pastors willingness to sacrifice their own kind. In the climactic confrontation, Heidi must fully emerge to battle the pastor parasite as it hops between host bodies. With help from a conflicted Jun-kyung, they destroy the parasite. In the aftermath, Jun-kyung covers up Su-ins existence, Kang-woo joins Team Grey, and Su-in attempts to return to her normal life while continuing to coexist with Heidi. A final scene introduces a self-proclaimed Japanese parasite expert named Shinichi Izumi, the protagonist of the original manga, who extends his right hand with his own parasite, Migi, hinting at a larger connected universe.