Episode 1 of Siren's Kiss begins in a factory where a worker is about to cut his fingers on the factory's machinery. Suddenly, a man in white shoes enters the room and tells him that the insurance claim papers are ready.
Next, we meet Cha Woo-seok, an insurance investigator who finds the man in white shoes at a barber shop and threatens to cut his fingers instead. As Kim, the man in white shoes, is arrested, news reports reveal that he was part of an operation that hired workers and then intentionally injured them to claim insurance payouts, with the middlemen earning billions in the process.
Woo-seok visits a hospital where a little girl named Chae-rin has been admitted for third-degree burns. A year ago, she was injured and lost sight in one eye. Currently, little Chae-rin tells Woo-seok that it was an accident; she dropped hot water on herself while making ramen. However, it's clear that the mother is using her child to get money, and Woo-seok is determined to prove it.
The scene shifts to an art gallery where Han Seol-ah storms inside and uses her knife to slash through one of the paintings. It turns out she's the chief auctioneer and has discovered that the painting is a forgery. She berates Kim Yun-ji, her associate auctioneer, for the oversight.
Back to Woo-seok, we learn about his key motivation for becoming an insurance investigator after being a cop. His sister Cha Woo-hee died in an accident with her boyfriend, Hyun-su, who was "coincidentally" the beneficiary of her life insurance. However, the cops, including Woo-seok's colleague Seong-il, couldn't prove it was Hyun-su and let him go.
At present, Seol-ah's auction is underway. A valuable piece called 'Universe' is being sold and won by a rich client named Paek Jun-beom. At the after-party, Seol-ah speaks to her friend Hwang Sook-ji who points out that Kim Yun-ji seems angry with Seol-ah. Indeed, Yun-ji has been downing champagne while throwing glares towards Seol-ah.
When Seol-ah steps outside for a moment, Paek Yun-beom follows and engages her in conversation. She claims to have seen him before at another auction. Soon, Yun-ji steps into the conversation, and Seol-ah leaves. This displeases Yun-beom, who crushes Yun-ji’s business card in his hand. Yun-ji joins Seol-ah in the dressing room, where another tense conversation ensues. Yun-ji claims to know Seol-ah’s biggest secret and promises to take her job soon. Seol-ah seems confident that this won’t happen. However, Yun-ji later makes a call to Cha Woo-seok, having heard that he’s relentless in his investigations. She tells him that someone’s been killed for an insurance payout. After the call, Yun-ji is suddenly attacked by someone in the stairwell.
Woo-seok and his colleague head out to meet Yun-ji. They have just reached the building when someone falls from the roof – it’s Yun-ji! The investigation into Yun-ji’s death kicks off, and the medical examiner says that it seems like someone grabbed her neck before she fell off the roof, which seems to rule suicide out. Detectives Gong Joo-young and Pyo Seong-il take charge of the investigation.
A quick flashback shows that it was Sook-ji who pushed Yun-ji in the stairwell. Woo-seok returns to the Royal Auction office and makes his way to the 12th floor, where he happens to see Seol-ah through a one-way mirror. Unaware of his presence, Seol-ah visits the company’s chairwoman, Kim Sun-ae, and promises to deal with the police investigation quietly.
Gong Joo-young and Pyo Seong-il question Seol-ah and don’t get much out of her. Afterwards, Seol-ah does seem shaken up and takes a pill to calm herself. As the cops are walking out, Woo-seok joins them and offers to work together. But he and Seong-il soon get into an argument and walk away, angry with each other. Nevertheless, Woo-seok learns about Seol-ah from Joo-young and starts digging into her.
At the funeral, Woo-seok overhears Yun-ji’s other colleagues talking about how the managing director, Mr. Song, was going to fire Seol-ah and promote Yun-ji instead. He sees Seol-ah leaving the funeral and follows, helping her out when her car keys don’t seem to work. Back in his office, Woo-seok looks up Seol-ah’s insurance details and learns that her fiancé, Yoon Seung-jae, had an insurance policy worth 700 million won and the beneficiary was Seol-ah. However, the policy was cancelled some time ago. Woo-seok makes a call and learns that Seung-jae died ten days ago, right after the policy was cancelled. At home, Seol-ah watches a video of her and Seung-jae in wedding attire.
Woo-seok gives all this information to Joo-young, who finds out that Seung-jae apparently died by drowning in a yacht accident. Woo-seok now approaches Seol-ah directly, being candid about his suspicions regarding both Yun-ji and Seung-jae. He points out that while Seung-jae’s insurance policy was cancelled, Seol-ah could still have benefited from the 200 million won premium that returned to Seung-jae’s account when the policy was cancelled. Seol-ah calls it a measly amount and coolly fields all his questions.
Meanwhile, Joo-young gets an anonymous phone call from a woman who disguises her voice as a man’s and tells Joo-young that Yun-ji did not kill herself; she was murdered. A final scene shows Seol-ah standing on the roof, watching Yun-ji’s body below, on the night of the murder.
