Soo-yeol and K shake hands on their new partnership, only for the electrifying BGM to fade off with a record scratch as K recalls something. Smile slipping off his face, K asks Soo-yeol why he did “that” — has Soo-yeol already forgotten about it?
We flashback to two months ago. Soo-yeol’s in a noraebang, receiving golf tips from a man that he’s trying to butter up. Turns out the man is someone that Gye-shik accused of doing drugs, so Soo-yeol’s here to assure him that he’ll take care of it. As Soo-yeol stumbles drunkenly into the hallway, he receives a call. The voice on the other end of the line tells him that Sang-ho didn’t murder Min-soo, and that he has the evidence to prove it. However, Soo-yeol’s too intoxicated to pay it any heed, heading back into the noraebang room instead. From afar, K watches with distaste.
In the present, K rebukes Soo-yeol for brushing the phone call off. Soo-yeol protests that he thought it was a prank call, to K’s ire — if Soo-yeol had caught Gye-shik back then, things wouldn’t have spiraled to this point. Determined to redress his mistake, Soo-yeol vows that he’ll make things right.
Based on their conjecture that the caller was someone in Sang-ho’s circle, the pair set off to investigate Sang-ho’s apartment. Soo-yeol takes particular note of Sang-ho’s freshman application documents to Munyang University, though we aren’t told their significance yet.
At the police station, Kyung-tae and Jae-sun have noticed something odd in the CCTV footage of Kyung-joon’s funeral. Jong-gu was absent at the same time that Hee-gyeom’s house was broken into, meaning he could have planted the drugs and fabricated the scene.
Having found a brochure for a drug addiction rehabilitation center in Sang-ho’s home, Soo-yeol heads there. The receptionist says Sang-ho wasn’t their patient, but she does recognize him as being Woo Hyuk-jin’s friend. Sang-ho had rushed into the center one night with Hyuk-jin on his back, yelling for help. Apparently, Sang-ho had always found and saved Hyuk-jin whenever he escaped from the center. Soo-yeol muses that Hyuk-jin must feel indebted to Sang-ho.
The receptionist tells Soo-yeol that Hyuk-jin can likely be found at the junkyard, so that’s where our pair heads next. Soo-yeol can barely see anything through his binoculars, so K suggests they move in. Having been burned by their previous encounter with Il-soo and his druggie minions, though, Soo-yeol is reluctant. K teases that Soo-yeol’s a druggie too, and that gives Soo-yeol an idea.
HAHA, a very disgruntled Soo-yeol exchanges his sharp suit for a drug addict’s ratty old clothes so that he can look the part. Soo-yeol recoils at the terrible smell, so K offers to swap out with him, only for Soo-yeol to realize that he’ll have to do it anyway since they’re one and the same.
Soo-yeol shuffles into the junkyard, only for a burly man to immediately beckon him over. Distrustful, the man manhandles Soo-yeol to check his forearm, only releasing him once he’s verified that there are track marks. (Good foresight, K!)
Soo-yeol manages to buy a packet of drugs from the man, only to run into a gang of druggies who accost him and steal the drugs. Upon a closer look, they realize that he looks familiar, so they demand that he injects the drugs into himself to prove that he’s not a cop.
Soo-yeol beckons for K to switch in, but K refuses to get involved. Getting increasingly frantic, Soo-yeol begins flailing at what looks like empty space to the druggies. Believing that he’s high, they laugh that he’s lost it and walk away.
Just then, Soo-yeol notices movement in a container truck. It’s Hyuk-jin, shaking from withdrawals. Soo-yeol asks if he knows Sang-ho, and the name rouses Hyuk-jin. Soo-yeol wheels Hyuk-jin out in a cart, only to run into the druggie gang yet again. This time, K takes them on, knocking a few out and running away from the rest.
They end up at Geun-tae’s office, where Soo-yeol attempts to question Hyuk-jin. Unfortunately, Hyuk-jin’s mind is too muddled by the withdrawals, so Soo-yeol ends up outlining a conjectured scenario for him — only for Hyuk-jin to blurt out that Soo-yeol’s got it all wrong. Not only did he not call Soo-yeol, but he also killed Sang-ho. What??
Two months ago, Hyuk-jin had run up to Il-soo to tell him that Sang-ho had left to report them and the corrupt cops that were in cahoots with them. Screaming in guilt upon remembering the memory, Hyuk-jin begins scratching his leg furiously.
Soo-yeol and Geun-tae rush to hold him down, and once Hyuk-jin’s calmed down, Geun-tae explains that such drug-induced psychotic episodes will eventually lead to Hyuk-jin’s death.
In an interrogation room, Il-soo’s busy rehearsing his sob story in front of Gye-shik and Chan-ki. Afterwards, he tells Gye-shik that his men had informed him of Soo-yeol visiting the junkyard the previous night and taking Hyuk-jin with him. When he explains that Hyuk-jin had ratted them out to Min-soo, Gye-shik flares up at him. Digging his fingers into Il-soo’s injured shoulder, Gye-shik orders him to mobilize his underlings in the women’s detention center.
At the detention center, Hee-gyeom is doing manual labor when a gang of menacing women saunter into the room. They move to attack her, and Hee-gyeom fights back, but she’s soon overpowered by their numbers. The minions pin her down while their ringleader presses a blade to Hee-gyeom’s neck. They don’t go further than that, though, instead leaving Hee-gyeom to tremble fearfully in the wake of their threat.
Incensed upon hearing of this, Soo-yeol storms into Gye-shik’s office to confront him. Gye-shik counters that Soo-yeol betrayed him first by digging into Min-soo’s case, then threatens to hurt the innocent people around Soo-yeol if he continues to cross the line.
Before Gye-shik can leave, though, Soo-yeol rises menacingly. Pointing out that Gye-shik couldn’t kill Hee-gyeom for fear of Soo-yeol flying off the handle, Soo-yeol realizes that Gye-shik’s rash counterattacks now mean that he’s on the right track with Min-soo’s case.
Realizing that Hee-gyeom’s stolen belongings must still be in Jong-gu’s car, Kyung-tae and Jae-sun arrive at the junkyard where Jong-gu’s car was abandoned. While they’re peering into the car, Jae-sun is attacked from behind by an unknown man, who swipes Jae-sun’s wallet from his back pocket. Kyung-tae immediately gives chase, and before long he’s handcuffed the man. Turns out the man had also stolen Hee-gyeom’s medal from Jong-gu’s car, which is the evidence they need to prove the break-in was fabricated and exonerate Hee-gyeom.
Outside the detention center, a concerned Soo-yeol (and an excited K) wait for Hee-gyeom. She gives a small smile upon seeing him, making K’s heart flutter. When Soo-yeol takes her bag for her and complains about how heavy it is, Hee-gyeom exasperatedly opens it up to reveal the countless letters K sent her, all with cheesy terms of endearment.
In Soo-yeol’s car, Hee-gyeom theorizes that the people who hurt her in the detention center were threatening someone else, since they didn’t kill her or say anything to her. Soo-yeol tries to change the subject, but Hee-gyeom catches on.
With no other choice, Soo-yeol pulls over, and they sit on a bench to talk. Soo-yeol admits to Hee-gyeom that there’s a mastermind behind the break-in incident — and that it’s Gye-shik. Reeling in shock, Hee-gyeom can’t believe that her teammates murdered Kyung-joon and that her trusted captain framed her.
Soo-yeol drops Hee-gyeom off at her house, then receives a call from Geun-tae informing him that Hyuk-jin’s disappeared. Before Soo-yeol can rush after him, he gets a call from Hyuk-jin, who asks if it’s possible to prove Sang-ho’s innocence by finding the informant who called Soo-yeol. Worried about Hyuk-jin’s safety, Soo-yeol insists that they meet up, but Hyuk-jin’s determined to find the informant on his own.
In a dark alley, Hyuk-jin meets up with Chan-ki. He asks Chan-ki if he was Soo-yeol’s informant, and we see in a flashback that Chan-ki had run off after overhearing the conversation between Hyuk-jin and Il-soo.
The next thing we see, however, is Soo-yeol approaching a crime scene far out in a field. Hyuk-jin’s corpse lies under a tree, dead from a drug overdose.
At a firing range, Chan-ki tries not to flinch while Gye-shik shoots round after round. Making veiled threats about how Soo-yeol’s digging into Min-soo’s case, Gye-shik hands his target sheet, riddled with bullet holes in the head, to Chan-ki.
In a flashback, we see that Chan-ki had driven Hyuk-jin out to the field, threatened to kill him if he saw him again, then abandoned him there. After Chan-ki left, though, Andrei had shown up. Later, Andrei reports to Boss Yong that Chan-ki hadn’t ratted out anything even till his death.
Night falls, and Soo-yeol confronts Chan-ki about his identity as the phone informant. He asks Chan-ki why he killed Hyuk-jin, and Chan-ki grits out that he didn’t. Soo-yeol offers Chan-ki an ultimatum — either hand over the evidence that will prove Sang-ho’s innocence and turn himself in, or rat Soo-yeol out to Gye-shik.
Grabbing Soo-yeol by the collar, Chan-ki bursts out that Soo-yeol should have believed him back then. Why do all this now? With a tired exhale, Soo-yeol admits that he’s tired of living a life full of regrets. Isn’t Chan-ki tired of it too?
Shaken, Chan-ki’s grip on Soo-yeol’s collar loosens, and Soo-yeol walks away.
The next day, Hee-gyeom goes to visit Gye-shik. She tells him about how she hadn’t been able to make sense of the incidents that had happened to her, because she trusted him too much. It was only after she entertained the possibility that Gye-shik was the mastermind that the pieces started to fall in place.
Finally revealing his true colors to her, Gye-shik chuckles that Hee-gyeom’s smart. Hee-gyeom’s devastated by this revelation, and she tearfully asks him how he became like this. In response, Gye-shik asks why they have to risk their lives to catch those dirty criminals in return for a measly paycheck.
As Gye-shik walks away, Hee-gyeom declares that the next time she visits, it’ll be to arrest him.
Chan-ki drinks alone in a bar, gazing dolefully at his bottle of whiskey. It was a gift from Min-soo, and we see in a flashback that Min-soo had paid off Chan-ki’s family debt for him. Guilt-ridden, Chan-ki finally calls Soo-yeol to ask if he’s confident that he can catch Gye-shik.
Chan-ki gets into a taxi to meet Soo-yeol at Min-soo’s columbarium, but he’s being tailed by Andrei, who snaps photos of him. Boss Yong calls Gye-shik, drawling that problem after problem keeps arising in their partnership. She sends him a video of Soo-yeol and Chan-ki’s confrontation the previous night, and Gye-shik speeds off in his car.
Having found out that the drug Pupil originated in Russia, Hee-gyeom now notices Boss Yong and Andrei in a commemorative photo with Assemblyman Do. Just then, her doorbell rings, and she checks the intercom to see a young girl wanting to offer rice cakes to her new neighbor. Naively opening the door, Hee-gyeom’s soon faced with a gang of thugs, who rush at her.
At the columbarium, Chan-ki hangs a bouquet of flowers at Min-soo’s niche. To his shock, Gye-shik arrives, commenting ominously that Chan-ki’s the last one of their team left. He demands for Chan-ki to hand the evidence over, and Chan-ki takes off running.
Soo-yeol arrives at the columbarium just in time to witness the chase. Gye-shik fires a warning shot into the air, then levels his gun at Chan-ki, which terrifies Chan-ki into submission. Pleading that he must have lost his mind for a moment, Chan-ki hands his phone over to Gye-shik. On it is a video recording of Gye-shik causing Min-soo to fall to his death.
Gye-shik accepts the phone gleefully, but ignores Chan-ki’s begs for forgiveness, kicking him to the ground. Venting his frustration on Chan-ki, Gye-shik rains blows down on him, only to be tackled aside by Soo-yeol.
Snarling at Soo-yeol, Gye-shik if he thinks he can catch him alone. With a confident smirk, Soo-yeol counters that he didn’t come alone. K’s pumped up and ready to fight.
Gye-shik pulls his gun on Soo-yeol, but K swiftly disarms him. In a series of smooth camera cuts, we see Soo-yeol and K working in tandem to take Gye-shik down. Soo-yeol confiscates the phone from Gye-shik, thinking he’s won.
However, Gye-shik cackles that by the time Soo-yeol takes him to the police station, Hee-gyeom will be dead. Stunned, Soo-yeol lets his guard down, and Chan-ki hits him from behind. Punching Soo-yeol repeatedly, Chan-ki wails that Soo-yeol shouldn’t have dragged him into this if he didn’t have the guts to finish it. He stomps on the phone, and Gye-shik crushes it under his foot, smugly advising Soo-yeol to go save Hee-gyeom.
Soo-yeol drives like a madman to the junkyard, rushing into the truck where an unconscious Hee-gyeom lies. Just as he’s about to carry her out, Gye-shik’s henchman locks the door, trapping them both, then begins driving the truck away. There’s a barrel of burning coal briquettes inside, spelling certain doom if they can’t escape in time.
Determined to get out, K continues kicking the door. Soo-yeol seems to have lost hope, until K calls him a softie — that’s enough to fire him up, and Soo-yeol joins him in smashing the door open. Coming to, Hee-gyeom stumbles to her feet, and together, the three of them kick the truck doors until they finally burst open.
Able to breathe at long last, Soo-yeol and Hee-gyeom hang onto the doors for dear life as the truck drives on.