A group of parasites have invaded the country possessing people . Once possessed, these parasites pretend to be humans living among them while secretly eating them.
Jeong su in is an ordinary girl who gets attacked and almost killed by a crazy customer she had a problem with at the store. She’s almost at the point of death when a parasite posesses her body and kills the crazy customer. But at the hospital, she is found to be fine leaving her detective protector Detective Chul min shocked.
Seol Kang woo is a gangster who failed to assassinate the current gang leader who has taken over the turf from his leader. He decides to lay low at his sister’s place, surprised when he discovers his younger sister is missing and his older sister who is supposed to be suffering from a brain tumor looks pretty hale and hearty. She’s acting very strange.
Meanwhile Jeong su in after that incident, begins to notice some changes in her, especially when she comes across certain people. Eventually, she gets approached by Kang woo’s older sister and a random man who have both been possessed by those parasites. The entity inside Su in reveals it’s self as one of them. Su in is then invited to their gathering at a church. When they leave the entity spots Kang Woo spying at them and threatens to kill him until he explains the situation to the human Kang woo.
When Su in regains her suspicions, Kang woo tries his best to tell her that her brain had been possessed by a parasite but not totally because she had been at the point of death when it entered. He warns her not to get near the other parasites for the sake of her safely. Then he leaves her in frustration to search for his sister.
Choi Jun kyung is the captain of the special task force anti parasyte team grey unit set up specifically to get rid of the parasites living among them. After the police officers at Namil station endure a rigorous check up, Choi Jun kyung introduces herself and tells them about the parasyte. How to identify it and kill it. Apart from an x-ray, the parasyte can be identified if someone plucks a hair strand from another and it withers away. The parasyte can only be killed by a direct stab to the chest using a knife or a shot gun.
Everyone is shocked when she brings out her husband who has been posessd by the parasyte and has a metal covering his head to stop him from harming anyone. Choi Jun kyung explains that her husband is a hunting dog who will help identify others like him. The police chief think she’s been inhumane to her husband but when Choi Jun knyung shows him her scarred hand and her plastic ear, everyone understands and she tells them her husband no longer exists and not to mistake a parasyte for a human or they would end up like her.
Meanwhile, the parasyte reveals itself to Su in. It writes to her in a book that it couldn’t fully take over her brain because she was nearly dead. It says it can only use her body for fifteen minutes especially when she’s in danger. It warns her to stay away from other parasyte because if they find out she’s only half like them, a mutant. She will be in danger. Su in is left in total disbelief after this.
Detective Chul min who is suspicious of Su in being a parasyte pays her a visit at the store. Under the disguise of catching up, he manages to take a strand of her hair before leaving. While Sun in is on her way home, she is met by the parasytes who immediately take her to see the pastor, their leader. Meanwhile Seol Kang who had managed to evade a group of thugs from the rival gang pursuing him, track down the parasytes to the church. He spots Su in among them wondering what she’s doing there after he specifically told her to keep away from other parasites. Before then, he had enlisted the help of his friend who promised to pick him up.
The parasytes hold a meeting with the leader addressing the need for survival especially now that detective Choi Jun kyung is on their tail. She would stop at nothing to find them and terminate them.
Meanwhile, Seol Kang had sneaked into the church’s warehouse trying to evade a parasyte who is keeping watch. But when he discovers his sister’s dead body in the warehouse, he looses all sense of control and attacks the parasyte thereby drawing the others attention. He flees but runs into the thugs who had traced him there. However, unfortunately for them, they end up becoming the victims of the parasytes.
Seol Kang woo uses the opportunity to escape but tumbles into Su in. The two decide to help each other and they both meet up with Kang woo’s friend who is waiting at their meeting point. They drive off But the friend is surprised that he’s alive. It isn’t long before Kang woo realizes that his friend had betrayed him by selling out his whereabouts to the rival thugs. The friend tries to kill him but the intervention of a parasyte overturns the situation. Su in transforms and fights off the parasyte eventually killing it before it can tattle tale to their leader.
Kang woo’s friend had been pierced by the parasyte and looses consciousness. Their car crashes close to a cliff. When the friend regains consciousness, he reveals that their boss ordered Kang woo’s death after joining hands with their rival gang. He dies. The car nearly crashes down but Su In’s parasyte saves him when Su in nearly dies trying to save him. It gets frustrated that it has to go through this just to stay alive.
Meanwhile, detective Choi Jun kyung looks through police report on Su in and easily identifies her as a parasyte.