A murder takes place on an evening in the pouring rain, and Cho Jung Hyun, who has just returned to her hometown after 12 years, realizes that Cho Jae Young, her beloved niece, has gone missing. Meanwhile, Choi Hyung In, the detective in charge of the murder case, finds out that there is a relationship between the missing girl and Cho Kyung Ho, the culprit who’s been charged and imprisoned for committing the Saju Station Gas Terror Attack.

Hyung In tries to find leads to Cho Jae Young’s case by listening to the cassette tape where mysterious sounds are recorded. When a drug addict’s body is found at the port, Hyung In realizes that this case is related to a drug ring. Meanwhile, Jung Hyun finds a videotape she watched with her high school friends. The content is so creepy and mysterious but it’s even more strange that no one remembers watching it.

Due to an explosion, Hyung In’s partner Si Jeong gets injured, which devastates Hyung In. This makes him drop the case. However, a key suspect of Jae Young’s case, Kim Hwan Kyu turns himself in. Jung Hyun hears from Jae Young’s friends that Jae Young had sent them a message a month before Kim Hwan Kyu turned himself in, and this makes Jung Hyun believe that Jae Young is still alive.

Jung Hyun is kidnapped and forced into a mysterious ritual. She realizes that Jae Young is not dead and is held by the people of Youngjingyo. The man who works as a representative of Youngjingyo reveals a shocking truth behind the 1987 terror attack. Hyung In also realizes that he’s been surrounded by Youngjingyo for a very long time.

After Jung Hyun is released and sent back home, she leaves again to look for Jae Young with Yong Tak. Another murder case occurs in Saju City. To get to the bottom of all this mystery, Hyung In decides to do it his way.

Hyung In hears from Jung Hyun that she saw Jae Young at the place where Youngjingyo believers hold their rite. When Hyung In meets Kyung Ho at the prison, he hears something shocking. Meanwhile, Jung Hyun is taken by mysterious men and meets Im In Gwan.

Jung Hyun turns down In Gwan’s offer and starts looking for leads with Yong Tak again. Meanwhile, Hyung In is put in a tough spot as Kim Hwan Kyu tells the press that he was tortured during the investigation. Hyung In also hears a rumour that there’s a video tape named “Mixed Tape” circulating in Saju City and that those who watched the video are saying strange things.

Hyung-in heads to the Cheonyang Filtration Plant to stop the disaster that is about to occur in Saju. But he becomes tied up by the Youngjingyo believers. There he hears that another gas terror attack that’s similar to that of 1987 is about to take place. The people in Saju start to flock around the filtration plant as if they’re possessed by something.

A few days after the Cheonyang Filtration Plant incident, Cho Kyung Ho escapes from the institution. Hyung In and Jung Hyun promise to meet up and go look for Kyung Ho together, but she doesn’t show up. She only leaves an address for Hyung In. When Hyung In gets there, he sees something he did not expect to see.

Jung-hyun joins Youngjingyo as the next heir who will replace Jae-young. Watching those inside who are merely waiting to face their death, Jung-hyun comes to the conclusion that she needs to take extreme measures. Meanwhile, Kyung-ho, after the release from the prison, pays Kyung-sook, his mother, a visit before the last phase of the experiment. Hyung-in at the same time realizes the intention behind Kyung-ho’s message.

By sacrificing herself, Jung Hyun shows the believers that what the proxy is saying is wrong, and the Youngjingyo believers change their minds. Meanwhile, Jae Young follows In Gwan to the Youngjingyo building to save her aunt, and Hyung In goes to Kyung Ho who is waiting for him. When they meet, Kyung Ho makes a final offer to Hyung In that can put an end to this long experiment.