Jo Pil-ho (Lee Sun-kyun) is an Ansan-based corrupt homicide detective whose money is tied up in a suspect real estate deal, with Internal Affairs breathing down his neck. Desperate, he robs a police warehouse with the help of Han Gi-chul (Jung Ga-ram). After Gi-chul breaks in, the warehouse explodes, killing him and leaving Jo unconscious.
At the scene, a fireman steals Gi-chul's phone and hands it to an unknown party for forensic recovery. The warehouse owner's body is found the next day in an alleged suicide. The fireman is also killed in a restroom. It becomes clear that the blast destroyed evidence implicating Jung Yi-hyang, the taesung Group CEO, in a multi-million dollar scandal. Gi-chul's phone contains footage of Kwon Tae-ju (Park Hae-jun) breaking into the warehouse before the explosion.
Waking up in the hospital, Jo is questioned by officers. As he denies involvement, Jo learns that Gi-chul was murdered before the explosion. Jo goes to Gi-chul's apartment where the two stashed their earnings, but the money has been stolen. He reviews CCTV footage and identifies Mina (Jeon So-nee), Gi-chul's girlfriend. At Mina's workplace, he encounters the father of Song Ji-won, Mina's close friend. Ji-won was a female officer who died in the Sewol Ferry incident.
Jo tracks Mina to a motel, where he cuffs her and So-hee (Kwon Han-sol) to his car and drives to Mina's address, ignoring Mina's claim that she received a video message from Gi-chul. Mina tazes Jo, causing a crash, and escapes with So-hee. Back at the station, Jo learns that Gi-chul sent the footage to Mina's phone before his death. Public prosecutor Nam Sung-sik urges Jo to retrieve the footage in exchange for clearing his name. To his relief, Jo finds Mina's phone in his car, but it is locked. He goes to Mina's address and finds part of his money stash, but runs into Kwon Tae-ju and is knocked unconscious just as Mina returns. Kwon prepares to kill Mina and pin the blame on Jo. Jo wakes, tackles Kwon, and they tumble from the apartment as the neighbors hear the commotion; Kwon retreats.
Jo drives Mina away, and she agrees to unlock the phone if he pays for So-hee's abortion at an illegal clinic. The abortionist pays him for one night with Mina, mistaking Jo for their pimp, and Jo savagely beats the man. Nam calls Jo, who reveals he has Mina with him, and agrees to deliver her as a key witness in exchange for a promotion. Jo takes the girls to his house and leaves So-hee in Hee-sook’s care, planning to bring Mina to the rendezvous point for testimony and a meal with Ji-won’s father afterward.
The rendezvous turns into an ambush, as Nam is on Taesung Group’s payroll. They torment Jo and threaten to kill Mina. Jo lies, offering the remaining footage in exchange for Mina’s safety, and Kwon releases Mina. But Mina’s mood is shattered by despair, and she jumps from the building. Jo is then taken to meet Jung Yi-hyang, the Taesung Group CEO, who offers him money for the video.
A short while later, on the day Taesung Group hosts a scholarship event, Jo visits the headquarters. He calls Internal Affairs officer Kim Min-jae to claim involvement with Taesung and dares them to catch him. Jo crafts a gun from non-metallic parts, intending to kill Jung Yi-hyang. His plan is uncovered; he is subdued, cuffed, and submerged in a soaking tub to drown. While Jung is addressing the students, Jo escapes, overpowers the guards, and retrieves his gun, fatally shooting Kwon. He then heads to the stage and shoots Jung Yi-hyang in the head, before Kim Min-jae fires at him.
An employee of Taesung Group steps forward to testify their role in destroying the evidence; Nam is also implicated. Jo survives the injuries and is charged with murder. Hee-sook adopts So-hee, and on the way to the trial, Jo hallucinates Mina smiling at him with friends.
