Original: 아무도 모른다
A high school boy falls off of the top of a building and lapses into a coma. A detective and his school teacher attempt to get to the bottom of the incident.
Young Jin lays her hands on a doll with six wings from a bereaved family member of the serial murder. She thinks that the doll has something to do with the serial killer. Her investigation leads her to New Life Church and a suspect. Meanwhile, Eun Ho sees a man collapse on the street and takes him to the hospital.
Sun Woo asks Eun Ho if anyone at school bullies him, and Eun Ho says no. Eun Ho receives a prize for saving a man who lost consciousness on the street the other day. Meanwhile, Young Jin finds it strange that the 9th victim Im Hee Jung’s cell phone is nowhere found.
Young Jin comes to visit Eun Ho’s patient room to check on him. She finds signs of assault on Eun Ho’s body, and that makes her think it’s a crime. She starts to follow the traces of what Eun Ho did on the day of the fall. Finally, she ends up in an abandoned building. Meanwhile, Sun Woo is certain that Dong Myung knows something about Eun Ho’s fall.
Sun Woo hears from Dong Myung that he used to work part-time at Millennium Hotel with Eun Ho upon request from Chang Su, Su Yeon’s boyfriend. Meanwhile, with Su Yeon’s permission, Young Jin goes into Eun Ho’s house to look for a bag Eun Ho used to carry. She catches Chang Su who tries to sneak into the empty house and hears from him about what Eun Ho had found at Millennium Hotel.
Dong Myung talks about how he became friends with Eun Ho and that he sensed something was going on between Eun Ho and Min Sung at school. Young Jin takes the security footage from the day of the cash dump in order to find out who left the money. Meanwhile, Hee Seob thinks that Jang Ki Ho is Im Hee Jung’s heir and starts finding Jang Ki Ho.
Young Jin’s team decides to investigate Lee Young Sik and Kevin Jung who are involved in the cash dump and could be Dae Hoon’s accomplice. However, Lee Young Sik is missing and cannot be found anywhere. Young Jin fails to get any detailed or crucial information from Kevin Jung. Meanwhile, Sun Woo finds out that the one Eun Ho saved on the street was Jang Ki Ho.
Young Jin gets upset after realizing that her team members didn’t tell her that they had found out about Jang Ki Ho. She starts to investigate the connection between Jang Ki Ho and Choi Dae Hoon. In the school, Sun Woo and Young Jin find Dong Myung and Min Sung together in the science lab, and Min Sung is bleeding.
Sun Woo goes to the abandoned building where Dae Hoon was found dead, but Lee Young Sik shows up out of nowhere and attacks Sun Woo. Then, Young Jin enters the building. Meanwhile, Dong Myung meets Tae Hyung in Eun Ho’s ward and tells how an old man asked him about Eun Ho and gave him a phone number the other day.
After talking to Sang Ho, Young Jin comes to know about Sun Woo’s background, and becomes sure that Ki Ho is behind Eun Ho’s accident. Later on, Young Jin visits Hee Seob to find more information about the day of the accident. Meanwhile, Sun Woo tries his best to protect Dong Myung as he is pushed into a corner, instead of being protected.
The truth of the accident gets revealed. On the day of the accident, Eun Ho runs away from Sang Ho after having a conversation regarding a book. After coming to know that Young Jin is on the rooftop of the hotel, checking the descending life line, Sang Ho tries to hide the evidence. Meanwhile, news regarding Young Jin’s relationship with Victim Number Eight airs, making things worse for her to investigate.
Both Young Jin and Sang Ho try to find Ki Ho. After coming to know that Eun Ho called Dong Myung’s mother before the accident, Sang Ho goes to talk to Dong Myung’s little brother, and comes to know the most important key to this case. Meanwhile, Young Jin gets suspended until the disciplinary committee regarding her case ends.
Sang Ho and Sun Ah get anxious and try to find the book before Young Jin does it. But since Jang Ki Ho is the only one who knows how to decipher the map in the book, they have to keep him alive. Meanwhile, Eun Ho starts regaining his memories little by little, and Young Jin and Sun woo try to find the book with his memories.
Sun Woo visits Eun Ho’s ward and learns that Eun Ho regained his memory. He also finds out that Eun Ho is Hee Seob’s biological son. Young Jin tells Sun Woo and Eun Ho to keep the fact that Eun Ho regained his memory a secret until she catches Sang Ho.
Sang Ho finally acquires the book, and tries to decipher the message it contains. Although Sun Woo lost the book to Sang Ho, he managed to copy the entire book and hand it to Jang Ki Ho through Young Jin. However, Jang Ki Ho isn’t sure what the message means.
Young Jin and her team arrest Oh Du Seok as the prime suspect of the murder of Yoo Pan Soo. Jang Ki Ho decides to lure Baek Sang Ho to the church where he has a one-on-one talk about the secret the book is holding.
Young Jin faces Sang Ho on the rooftop of the hotel where he’s holding Eun Ho hostage. Sang Ho provokes Young Jin by talking about Su Jung’s last moments and the words she left. Young Jin has to try hard not to break down and capture Sang Ho alive.
