In the fourth episode of Siren's Kiss, we are introduced to a flashback where Yun-ji watches Seol-ah from her secret apartment. At the present time, Woo-seok realizes what Yun-ji was doing and discovers a burner phone inside the toy house. Meanwhile, at the police station, Eun-hye tells the detectives that she saw Seol-ah coming down the stairs from the rooftop, looking scared.
Seol-ah organizes a charity auction for emerging artists, where a man named Park Min-cheol, the owner of Taemyung Art Centre, bids on five pieces. A quick flashback reveals that Chairwoman Kim advised him to buy those pieces. It becomes clear that Chairwoman Kim has planted someone in the audience to compete with Park and raise his bid. However, at the end of the event, the detectives arrive and arrest Seol-ah for Yun-ji's murder.
Woo-seok watches this unfold and gives the burner phone to a hacker (a young man he caught trying to commit insurance fraud), who tells him he found spyware on it - Yun-ji was spying on Seol-ah. At the police station, Seol-ah admits to going to the roof but says Yun-ji was already dead by the time she got there. She ran away because she was scared. However, Seong-il makes it clear that her lack of alibi is an issue.
Elsewhere, Jun-beom sees a news article about Seol-ah getting arrested and smiles. Eun-hyuk visits Seol-ah and asks her if she saw anyone else on the rooftop. She whispers something to him, and Woo-seok takes a video of this from outside the interrogation room. Later, Eun-hyuk rushes to a gallery at the auction house but Woo-seok has beat him to it. Turns out, he got a lip reader to interpret Seol-ah's conversation with Eun-hyuk and then followed her instructions till he found a memory card.
The memory card holds video footage from an artwork by an artist named Hwang. It's the same passage with the one-way mirror that Woo-seok first saw Seol-ah in. The video footage shows Seol-ah walking through the passage at the time of Yun-ji's death. After seeing this, the detectives' higher-ups tell them to close Yun-ji's case as a suicide. Seol-ah is let go!
Woo-seok meets the hacker and sends him the audio files he found on the burner phone. Woo-seok then asks him to find Hyun-su (his sister's boyfriend). The audio files reveal that Yun-ji was listening in on Seol-ah's daily life. Woo-seok goes back to the apartment and notices that the toy house has figurines of a woman and a man, and the man is a doctor.Meanwhile, Seol-ah finds it impossible to sleep and visits her doctor. The doctor informs her that prescribing more tranquilizers for her insomnia is not an option and suggests she see a psychiatrist. Seol-ah shares her distressing experience at a mental clinic, and quick flashbacks reveal Choi Young-ho inappropriately touching her.
Woo-seok meets with the nurse from Choi Young-ho's clinic, and she confirms that Yun-ji was his girlfriend. He broke up with her shortly after Seol-ah began her consultations, which she took badly. After two months of sessions with Seol-ah, Young-ho stopped them. The nurse speculates that this was because he wanted to be in a relationship with Seol-ah.
Elsewhere, Park Min-cheol meets Seol-ah and tells her that Chairwoman Kim's advice worked out well for him. At a gallery, Woo-seok meets Seol-ah and tells her the truth about Yun-ji. However, this isn't an act of compassion; he still wants to know why her three exes had to die. She says she didn't gain anything; she only lost. As she's walking away, Seol-ah suddenly passes out. At the hospital, the doctors say she's taken too many tranquilizers and needs rest.
But when she wakes up, she leaves without saying a word to Woo-seok. She goes straight to Jun-beom's office, where a painting of flames triggers her childhood memories. We see a young Seol-ah, a burning house, and what looks like the dead bodies of her parents. Jun-beom refuses her proposal for the Stiller and says he's more interested in her. She tells him off for mixing personal and business issues, and he pacifies her, saying he'll stick to business talk.
One of the audio clips Woo-seok hears is a conversation between Seol-ah and Sook-ji. The latter berates Seol-ah for bottling up all her pain. Seol-ah says she doesn't deserve to be in pain since she survived but her parents didn't.
Woo-seok approaches Eun-hyuk and asks him about Seol-ah's parents. He remains vague about it, but Eun-hye arrives and tells Woo-seok that Seol-ah's parents died in a fire when she was 15. He looks through the official report, where it says the fire may have been started by someone wanting to commit suicide.
The next day, Seol-ah attends Seung-jae's enshrinement ceremony, where Seung-jae's mother blames Seol-ah for his death. Seung-jae's brother confronts her as well and demands to know why Seung-jae took out an insurance policy. It turns out he overheard Yun-ji warning Seung-jae about Seol-ah and the insurance policy. When he grabs Seol-ah and asks if she killed them, Woo-seok (who has been watching for a while) intervenes and makes him stop.
Seol-ah throws the engagement ring Seung-jae gave her off the side of the mountain. Woo-seok approaches her again and asks if she's really innocent. She steps back and begins to fall over the cliff but he pulls her back. She begs him to let her die and asks what he wants from her. He says it's her. He wants to know more about her.
