When unidentified parasitic spores fall from the sky, they begin taking over human hosts by entering through the ears, nose, or mouth and consuming the host's brain, completely erasing their consciousness. The parasites can transform their heads into lethal bladed tentacles to hunt and devour humans for survival. The South Korean government forms a covert special forces unit called Team Grey, led by Choi Jun-kyung, whose husband was killed by a parasite, to track and eliminate these monsters.

The story centers on Jeong Su-in, a grocery store cashier with a traumatic childhood marked by abuse from her father. After an altercation with an angry customer who follows her home, he stabs her repeatedly and leaves her for dead on a deserted road. At the moment of her near-death, a parasite enters her body. Because Su-in was critically injured when the parasite invaded, it fails to take over her brain. Instead, it heals her wounds, creating a symbiotic coexistence where both human and parasite share one body. Su-in names her parasite Heidi, and Heidi can only take control for fifteen minutes per day, typically activating only when Su-in is in mortal danger.

Parallel to Su-in's story, Seol Kang-woo, a low-level gang member, is hiding from a rival gang after a botched assassination attempt on a gang leader. He returns to his hometown searching for his missing younger sister, discovering that his older sister has been taken over by a parasite and is now part of a parasitic organization gathering at a place called Saejin Church. Kang-woo crosses paths with Su-in and Heidi, and the two become reluctant allies.

Team Grey investigates a mass killing at an EDM festival where a parasite slaughtered dozens of people on camera. Captain Choi Jun-kyung reveals that the parasites dropped worldwide months ago and are now organizing into groups. The team discovers that some parasites have formed a pseudo-religious organization at Saejin Church, where they coordinate and recruit. Detective Kim Chul-min, the officer who rescued Su-in from her abusive father years ago, becomes entangled in the investigation and protects Su-in from both Team Grey and the parasites who view her as an anomaly.

As the season progresses, the parasites' leader, Pastor Kwon, betrays his own kind in a bid for power, planning to infect the town mayor and seize control of human society. Su-in and Heidi learn to cooperate, their two consciousnesses gradually finding common ground despite their fundamentally different natures. In the finale, an alliance forms between Su-in, Heidi, Kang-woo, Kang-woo's infected sister Kyung-hee, and Team Grey to stop Pastor Kwon's plan. Captain Jun-kyung faces a moral dilemma when she must choose whether to trust Heidi or an infected police officer, ultimately deciding to spare Su-in and keep her hybrid status a secret.

After the crisis resolves, an investigative journalist arrives at Team Grey headquarters offering assistance in understanding the parasites' origin. This journalist is Shinichi Izumi, the protagonist from the original Japanese manga, now an adult who has continued studying the parasites he once fought alongside his own partner, Migi. His appearance establishes the shared universe between the Korean adaptation and the original source material. Su-in accepts her coexistence with Heidi, and Kang-woo joins Team Grey, setting the stage for continued battles against the parasitic threat.