Hyung Bin who is a detective asks his girlfriend, Ha Eun to marry him. But she refuses because of her heart disease. Meanwhile, another dead body has been found. Some crazy woman rushes into the crim scene and picks up a yellow umbrella. And the mystery man also has a yellow umbrella with him.

Ha Eun teaches kids at the church. By chance, Ji Chul takes her class and he ends up falling in love with Ha Eun. One day, Ji Chul sees an abused child who used to be his friend dead. Ha Eun collapses from her heart disease and Ji Chul finds out that she needs a heart transplant.

Ha Eun collapses after she witnesses a murder scene. And Hyung Bin finds out that it was Ji Chul who brought Ha Eun to the hospital. Hyung Bin protects Ha Eun day and night after the incident, and Ji Chul always watches Ha Eun from distance.

Jong Bum runs into Sa Bin at the University and he finds out that she had a heart implant when she was young. Sa Bin hears rumors about Jong Bum from students that he's actually a psychopath and he has killed someone. Su Hyuk and Sa Bin encounter at the place where some remains are discovered.

Jong Bum goes home but his father and brother don't seem to like him at all. The mummy found from Donggang River turns into skeletons in a day. Su Hyuk keeps feeling pain in his eye and Jong Bum's hand keeps twitching. And finally, Sa Bin, Jong Bum, and Su Hyuk meet.

Su Hyuk tries to forge the evidence of Donggang River mummy so Byun Sung Hoon can be sentenced to life. Jong Bum visits a woman's house to do the errands for her and she asks him to have a drink with her. Sa Bin and her friend try to restore the face of a mummy from Donggang River.

Su Hyuk finds out that it was Jong Bum who was with the victim of a copycat murder case. But Su Hyuk feels pain in the heart as he follows Jong Bum to the exhibition and sees him with Sa Bin. The criminal Byun Sung Hoon presses Su Hyuk with the lawyer. Sa Bin discovers crucial evidence from the book that was with the mummy and calls Su Hyuk.

Su Hyuk saves Sa Bin from a terrible car accident. Sa Bin's heart starts to pound for Su Hyuk. Su Hyuk tells Jong Bum's father that Jong Bum is the prime suspect of a murder case. Jong Bum decides to leave his family.

Sa Bin posts the restored image of a mummy from Donggang River. Su Hyuk finds out that Sa Bin posted the image and gets furious. Ji Chul picks up the notebook that Sa Bin dropped and reads about Gong Ji Chul. Su Hyuk makes a deal with Seo Tae Ha and looks for the buried bodies that were killed by Byun Sung Hoon.

After the remains of a buried baby are found, Jong Bum and Su Hyuk face each other as a prime suspect and a prosecutor. Jang Hye Mi, the writer of "Secrets of A Murderer," says that Gong Ji Chul was never a psychopath. She is sure that Jong Bum is going to kill someday.

It turns out that the remains were identified as Lim Hwa Young's baby. Sa Bin realizes Jong Bum's relationship with his little brother and sympathizes with him. Sa Bin and Su Hyuk visit “Old Future,” the secondhand bookshop. Both Su Hyuk and Jong Bum begin to have feelings for Sa Bin.

Su Hyuk lost his father who believed in humanity in the past. After his death, Su Hyuk has been fighting against beast-like criminals. Jong Bum’s feelings for Sa Bin grow bigger. But he finds out that Sa Bin has feelings for Su Hyuk, and a post is uploaded on Seoyeon University’s social media site.

Sa Bin sees the painting of Cain and Abel and has a hunch that something will happen with Jong Bum and his little brother, so she follows him to the charity gala. Sa Bin almost drowns and as she's close to death, she remembers a strange memory. Jong Bum dreams about killing Sa Bin, whom he loves.

Su Hyuk investigates what happened on the day Jong Woo almost drowned and finds out Jong Bum was at the lake before the incident. Meanwhile, Jong Bum shows up where Sa Bin is using the tracking application. Su Hyuk visits the nursing hospital where the prison officer who executed Gong Ji Chul is, but it turns out that he is missing.

Sa Bin struggles with her feelings for Su Hyuk. Jong Bum decides to get rid of Su Hyuk for Sa Bin and himself. Chief Prosecutor Chun warns Su Hyuk to leave Jong Bum alone, but Su Hyuk makes up his mind to go against him. Sa Bin finds out that a woman gave her father a note.

Sa Bin gets transferred to the hospital after she was stabbed by a thug. Su Hyuk finds out that it was Jong Bum who incited to murder Su Hyuk. Sa Bin notices that there is a tracking app on her phone and she assumes that Jong Bum was following her all along. The police discover a tattoo from the victim, Lim Hwa Young, and start to track down the person who purchased the tattoo ink.

Jong Bum takes a picture of Detective Ju being attacked by someone. And he meets and talks with the attacker. Su Hyuk says goodbye to his fiancee and confesses his feelings for Sa Bin. Su Hyuk discovers that Jong Bum’s father was the prosecutor in charge of Gong Ji Chul’s murder cases. Jong Bum sees Sa Bin and Su Hyuk kissing at the bookstore and suddenly feels pain in his eye.

Su Hyuk sees Jong Bum on the camera which was installed at the place where Detective Ju was attacked. So Su Hyuk believes Jong Bum attacked Detective Ju. Jong Bum notices his house was robbed by someone and realizes that his painting was missing, and all of his photos on his phone are sent to somewhere else.

Jae Yi tells Su Hyuk that Jong Bum has been stalking Sa Bin. And Su Hyuk is convinced that Jong Bum attacked Detective Ju after hearing the recording file from Detective Ju’s phone. Jong Bum invites Sang Ah, telling her that he’ll show his first murder, and Sang Ah invites Sa Bin to the place.

Tae Ha takes Jong Bum to the hospital. Jong Bum’s mother falls into despair as the doctor tells her a bullet is in Jong Bum’s head and that his brain might be damaged. There has been a rumor that Jong Bum went to America, but a student comes up to Sa Bin and tells her that she saw him near Insa-dong.

Finally, the remains from Donggang River are restored with a hologram. But as Su Hyuk sees the hologram, he collapses with unknown memories and pain. After he opens his eyes, it seems like Su Hyuk has become someone else. Jong Bum is back to his family. He doesn’t remember anything but his family doesn’t seem to notice.

Feeling confused, Hyung Bin heads to Old Future. He asks the owner where Ha Eun was buried. After Jong Bum opened his eyes, he introduced himself as Gong Ji Chul in the test he had at the hospital. A male remains are found near in the East Sea, Gangwon-do. And JI Chul encounters the remains at the NFS.

Ji Chul tells Hyung Bin that the real killer is Gong In Woo. Hyung Bin finds the killer’s paintings from 30 years ago. Ji Chul visits Hye Mi and threatens to bring Gong In Woo. Hyung Bin finally gets to where Ha Eun was buried. Sa Bin gives the ring back to the owner. And again, Hyung Bin, Ji Chul, and Gong In Woo head to the same place.

Gong Ji Chul and Gong In Woo finally meet. From their conversation, Hyung Bin realizes that it was Gong In Woo who killed all those women in the past. Meanwhile, the face of the death-row convict who was found by the sea is completed. But it looks just like Jong Bum. Another body has been found, and Hyung Bin notices something.

Sang Ah is taken to the police as a suspect of the Yellow Murder Case. Hye Mi presses Suk Tae to let Sang Ah to be released. Ji Chul threatens Hye Mi with the identification card of her stepfather. But it seems like Ji Chul is out of his time. Su Hyuk tells Sa Bin that he’ll go back to her after sorting things out, but Sa Bin can’t let Jong Bum alone.

Sang Ah goes to trial and Detective Ju testifies that it was Sang Ah who stabbed him. But Hye Mi denies all charges as her lawyer. Seok Tae keeps trying to stop Su Hyuk from digging into Ji Chul’s case, but Su Hyuk is determined. Sa Bin shows a restored image of the death-row convict to her father and her father falls in shock.

Su Hyuk realizes someone broke into his house and took files related to his past cases. Sa Bin receives a message about his father and hurries to the hospital. In Woo tries to kill Sa Bin, but luckily Jong Bum comes on time to save her. The owner of the “Old Future” hands a recorder to Sa Bin.

Seok Tae gets into trouble after Gong In Woo was arrested for the Yellow Umbrella Murder. As the crisis deepens, Hye Mi presses Tae Ha to take care of Su Hyuk. So Tae Ha decides to put an end to all. Sa Bin wakes at the hospital but she seems rather strange. When Su Hyuk comes to see her, she jumps out of her skin.

The broadcast about Gong In Woo goes viral and the photos of Gong In Woo and his victims are sent to Su Hyuk by someone. Sa Bin feels lost in between the feelings for Su Hyuk and Jong Bum. In Sang Ah’s trial, the photo handed by Jong Bum acts as crucial evidence and she gets sentenced for life. Now, Jong Bum heads to solve his mystery.

Su Hyuk submits his resignation and visits Tae Ha to apologize. He suggests for the retrial for Tae Ha’s brother. Sa Bin tells Jong Bum that it was Seok Tae who killed Ji Chul in the past. Donggang Mummy is identified as Hyung Bin, and Sa Bin brings the remains to Su Hyuk. But what Su Hyuk confesses makes Sa Bin confused.

Sa Bin collapses after days of crying. Jong Bum can’t stand watching her sad so he makes up his mind to save her. Sa Bin gives the ring back to Su Hyuk saying her heart now belongs to Jong Bum. The time is running out for Jong Bum so he asks Tae Ha his plan.