[2] The plot follows two detectives, a veteran male cop (Song Kang-ho) and female rookie (Lee Na-young), who are appointed to investigate serial murders.

Middle-aged police detective Jo Sang-gil (Song Kang-ho), a single father with a young son and daughter, is assigned the seemingly straightforward case of a man, Oh Gyeong-il, who set fire to himself inside a car.

Chafing at his lack of promotion after so long on the police force, but under pressure from his boss-cum-friend (Shin Jung-geun), he also grudgingly agrees to work with rookie detective Cha Eun-young (Lee Na-young), a 30-year-old divorcee who has just been transferred from motorcycle patrol duty.

Sang-gil traces the sex club the victim had visited but bawls out Eun-young when she makes an amateurish error during a fight.

Eun-young starts questioning police-dog trainers, as well as investigating those missing or dead, and comes up with the name of Kang Myung-ho, who supposedly committed suicide a while ago.