The beginning shows Jin hee, Kang woo’s sister getting ambushed by her older sister and the pastor’s parasytes before getting killed. Now, after being on the run, Kang woo calls one of his close friends Ki seok hoping to lodge at his place. His friend easily agrees, giving him the password to his apartment. Without wasting time, he and Su in immediately head over there.
Meanwhile, the higher ups have been informed of Jin hee’s disappearance and aren’t happy about it especially when an important music festival to honor the town’s mayor is about to take place. The police suggest the festival be postponed but an important government official highly goes against it. Instead, they suggest that they let the public know about the parasites so that they can help provide any information on it.
Woo jae and Chul min have their own agenda and soon after, they approach Captain Jun kyung to tell her about a hidden parasite dwelling. They even provide video evidence showing that it’s the exact place where all the parasites gather. With their advice, Sun kyung agrees to raid the area.
Meanwhile, Kang woo and Su in are now in his friend’s apartment and Kang Woo expresses his desire for them to run away but Su in refuses, telling Kang woo that she would try to get captain Jun kyung on their side. Kang woo tries to stop her pointing out that Jun kyung would never believe her. He turns on the TV and when he sees the news talking about the parasytes and enlisting the public’s help to identify them, he feels validated but Su in doesn’t listen. That night, she steals some money from his wallet while he’s asleep and leaves but unknown to her, Kang woo is awake but makes no effort to go after her.
Su in then calls Jun kyung for a meeting. The two talk on opposite roof tops and Su in reveals that Chul min and Won seok are both parasytes. Jun kyung doesn’t believe her especially since she was talking against Chul min who had tried to protect her. But Su in tells her that Chul min died in front of her and that the mission they are about to undertake is a trap but Jun kyung doesn’t listen to anything she says and before she knows what’s happening, she is surrounded by the police. To save herself, Heidi takes over her body and escapes.
Meanwhile Kang woo is burdened by detective Chul min’s words calling him a coward. He then has a change of mind after learning from his friend that Jin hee had contacted him before she went missing comes for Su in in his friend’s car and drives off. On their way, Heidi tells Kang woo that for the next three hours, Su in shouldn’t be put in any danger. She had exhausted her energy and needed to sleep to regain her strength. if Su in happens to be in a dangerous predicament, she wouldn’t be able to help until after three hours. Heidi collapses and when Su in takes over, Kang woo tells her what Heidi had said to him.
But they are caught in a helpless situation when they receive a call from Kang woo’s friend Ki seok telling him his older sister is in his house. Kang woo panics knowing that is a parasyte and not his sister but before he can warm Ki seok, his sister attacks him. She then picks the phone and threatens him to bring Su in to her or she would kill his friend.
Despite Kang woo’s protests, Su in insists on going. She arrives at the apartment alone and tries to reason with Kang woo’s sister but the other woman can tell that the parasyte inside her is sleeping and tells iy to come out or she would die. But Kang woo suddenly arrives attacking his sister with a fire extinguisher to distract her but in the process of trying to escape, his friend Ki seok is attacked. To enable them leave, he sacrifices himself and dies.
Kang Woo is beside himself with anger vowing to kill every last one of the parasyte. While driving away, they call captain Jun kyung to warn her of the parasites trap but she doesn’t believe them especially after secretly cutting Won seok’s hair and discovering he’s not a parasyte but when Kang woo reveals that Won seok is the one who killed her husband, she’s a little shaken up. She then tells them to record everything that happens. If she ends up dying in the mission, they should take the recording to her superior.
But to their surprise, the mission goes well and all parasytes in the perimeters are killed. Jun kyung speaks to Kang woo and Su in on the phone telling them they were wrong in a mocking manner.
Meanwhile, Kang woo’s sister who is coming back on a bus sees the pastor who is now Chul min killing their kind. She has discovered he has his own agenda and feels betrayed for the first time.
Now, Kang woo and Su in are left to ponder what the pastor’s real motive is. Kang woo after seeing a poster of the mayor on the wall finally figures it out. What the pastor really wants is to take over the mayor’s body during the festival.
Episode six takes us a little further back to when Woo Seok partnered with pastor’s parasite. He had felt deflated by his wife who looked down on his job as a police man so when the pastor who had murdered his own wife offered him a deal to help him possess the head of the mayor, he did not refuse.
After Chul min’s speech, announcing that Su in and Kang woo’s sister are wanted people, captain Jun kyung approaches him, suspicious of his motives given the fact that he used to be protective of Su in but Chul min lies that he simply wants to protect the public. While leaving, Chul min suggest they kill Jun kyung since she’s suspicious of him but not now, until their plan has been realized. Meanwhile, Kang woo’s sister calls him and Su in suggesting they partner up to kill the pastor parasyte. Kang woo refuses to help but Su in is more receptive.
When they arrive at the meeting spot which is a park, Heidi takes over and meets with Kang woo’s sister but Kang woo filled with revenge tries to kill her but his sister easily thwarts his plans saying that now she understands how he is feeling which only makes Kang woo mad. She then tells them that the parasite who gave them their identity betrayed them and that they should join hands in killing him with her. Kang woo responds angrily that after killing their leader parasite, he would kill her too but his sister doesn’t mind and concludes that until then, they are partners.
Captain Jun kyung’s suspicions is further heightened when she recieves orders from her superior to go to another town called Gangwan do where a group of parasytes are dwelling. This tip off came from Chul min. Jun kyung has no choice but to obey but changes her mind when Heidi calls her to inform her that they are on their way to kill the parasyte inside Chul min whose purpose is to take over the mayor’s body.
At the festival, everything seems normal after the mayor’s arrival. Su in, Kang woo and his sister don’t find anything amiss, that is until the parasite inside Chul min goes on a rampage and directly attacks the mayor. The mayor’s guards struggle to protect him but eventually lose him in the chaos so Won seok under the guise of protecting the CEO takes him to a private hall where the parasite having ejected from Chul min’s body kills the remaining guards protecting the mayor. The parasite then tries to kill him but it’s stopped by Kang woo’s sister. They engage in a fierce battle while Kang woo uses the opportunity to protect the mayor. But the fighting has left Kang woo’s sister weak but instead of taking his body, she decides to protect him by pushing him and the mayor away. She apologises to Kang woo before the parasite stabs and kills her.
Meanwhile after landing at the festival, captain Jun kyung and her men immediately seperate. She ends up facing off against Heidi but she’s defeated by her. Heidi decides not to harm her. She runs off to protect the others. Meanwhile, having found no other living human host, the parasyte cuts off Won seok’s head and takes over his body, then he starts going after Kang woo and the mayor. Kang woo manages to protect the mayor but the parasyte gets a hold of the mayor, nearly killing him until Heidi appears giving Kang woo and the mayor the opportunity to run to safety. They make it out and Kang woo leads the mayor to his men waiting with his car. After the mayor leaves, he goes back for Su in.
Su in and the parasyte fight it out with the parasite sneakily dividing itself into two parts. Won Seok decides to stop fighting and starts telling a speech to Heidi trying to get her to join him. Just then, Jun kyung enters pointing a gun at Heidi unaware that the other parasite is getting ready to kill her. Meanwhile, Won Seok tries acting human, shouting at Jun kyung to kill Heidi but she notices that parasite behind her and in a split second shoots Won seok while Heidi kills the other parasite.
Won Seok dies and Jun kyung asks Heidi why she saved her but Heidi replies that she was only copying Su in who still trusts humans despite all she has been through. She says she will no longer possess Su in and that Jun kyung should look after her. She collapses and just then, Kang woo rushes in crying for Su in as he holds her in his arms. The police surround them but Jun kyung orders them to leave Kang woo and Su in alone saying they are both humans.
In the next scene, we see Su in returning to her normal life working at a supermarket. Kang woo pays her a visit and the two catch up with each other as he reveals that he was now a part of Team Grey and that Jun kyung also wants her to join them. He then gives her a letter Heidi had sent him telling her that she isn’t alone. Su in smiles as Kang woo waves at her and leaves.
In the next scene, we see Jun kyung in her office meeting with a strange japanese man who claims to be an expert with the parasytes. The series ends as he introduces himself and extends a hand towards Jun kyung. (Not sure what it means but well…it’s over)