NIS profiler Seo Dabin and fellow agent Oh Sehun are—secretly—on vacation together, visiting an island off the coast of Busan. Dabin has recently solved a notorious cold case, so she considers their holiday a reward of sorts. More than that, it's a cautious acknowledgment of her growing feelings for Sehun.
One day a body washes up on the beach, a body that's been disfigured. The killer has carved the word '' into the victim's stomach. A second murder, with the victim labelled a 'killer' appears soon after. The police don't agree when Dabin tells them the murders look like the work of a vigilante trying to avenge…something.
Dabin and Sehun find their own lives at risk as they move closer to the truth.
She kept quiet about it for a good two hours, through swapping stories at Bubba's Bar, a snack shack on the pier, and she almost made it back to their condo before she burst out, "That crime was personal."
"I agree," Sehun said. "But you're not on the case. And they've only got one victim, so this isn't an investigation you'd get called in for anyway."
"Probably not. But what if this is what I saw yesterday?"
Sehun slowed to a stop, studying her. "What do you think you saw?"
"A small boat. Something going overboard."
"This guy was shot. Which means if that's what you saw, he was most likely already dead when the killer took him out in that boat."
"Yeah," Dabin agreed, frustrated. "But if I'd kept watching, maybe I would've spotted the killer."
"The police will find him," Sehun said, walking again, swinging their linked arms. "Come on. We've only got a week. You deal with deranged killers all day, every day. Give someone else a turn."
Dabin grinned as she looked over at him. "Deal."
But as they reached their condo, Sehun swore and muttered, "Or maybe not." In his voice she heard anger, protectiveness and resignation.
Sehun pointed, and Dabin looked at their door. Stuck to the centre with a hunting knife was a note.
"You should understand this more than anyone. Let it go, SA Seo."