Mapo District, Seoul — Jo Sang-gil, Song Kang-ho, a seasoned detective and a single father to a young son Lee Tae-ri 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, he 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. The dead man, who had drugs in his system, has a large dog-bite on his thigh but no fingerprints. The police discover the immolation was not suicide but was triggered by a timer in the victim's trouser belt. Sang-gil traces the sex club the victim had visited but bawls out Eun-young when she makes an amateurish error during a fight. Against protocol, Sang-gil omits to file reports on the progress of the case, as he wants to earn all the kudos himself and thereby a promotion. His boss finds out, just as a second victim, Nam Sang-hun, is found bitten to death by a large dog or wolf. After talking to dog-trainers, the police learn the man they need to see is Min Tae-shik, a retired police dog trainer who lives with his drug addict daughter; during a raid on his house, Min escapes and a wolf-dog subsequently kills a woman in the backstreets. 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. She sets off to investigate on her own.
After 4 people fell victim to the dog, Eun-young investigate Tae-shik's home. From the house she discover he has been training his dog, Jil-Po, to kill 6 targets, including a police named Bae. She found Jung-ah in a hospital and gather more information where found that Jung-ah was used as prostitute by the 5 victims. We then switch to a corrupt cop and Min, a businessman who paid him to kill Oh Gyeong-il before. Their deal went awry and the businessman kill him, make it look like a suicide and left a fake last will. Police Department freeze the case after their death but Eun-young still bothered by the unknown 6th man.
Police Department later declare that they will start a hunt to kill the dog. Eun-young and Sang-gil use this information to then convince Young-jin to tell them who is the final target, which is revealed to be a minister. Noticing that all 5 of his henchmen are dead, the minister remember that Jung-ah used to tell him about his dad who was a dog trainer. He then plot his escape using boat. Eun-young later demoted to patrol police by their chief.
The police department then set up an ambush using Jung-ah's cloth in their old house. The ambush fail and Eun-young follow Jil-Pong to an old house where the minister prepare to escape. During the ensuing fight Sang-gil shoot and kill Jil-Pong while he bites the minister, which he later explain that he missed. The movie end with Sang-gil rewarded for the case and Eun-young help to rehabilitate Jung-ah.
