On a cold autumn day in September, chief detective Jung Daehyun is summoned to a gory crime scene on the outskirts of Seoul. The first thing he notices is a card lodged within the victim's wound, smudged words reading ready or not, here I come. As his team and him gather more baffling evidence with no traces of the murderer's identity left behind, Daehyun feels a choking sense of foreboding rising in him. The lack of substantial clues, the degree of precision, the odd remnants—this is no ordinary murder.

As the victim is flipped around, another card stuck to her back is gingerly removed. You didn't even try to hide, you're no fun. Youngjae returns from his inspection and collects the last of ten number cards, forming what seems to be a countdown, and turns to Daehyun grimly.

They're dealing with a potential serial killer here.