A barefooted Seo Yul (Ahn Se-bin) gasps for air as she runs past the peripherals of the apartment complex. She takes refuge amid its recycling section, the cluster of recycling bags hiding her from sight, but winces slightly when she pulls down her shirt sleeves. Heading towards a concealed flower pot, Yul lifts up the bag covering it – only to find the pot empty.
After narrowly being discovered, Yul crawls out from behind the recycling bags and comes face to face with counterfeiter Cheon-won (Jang Won-young), who taunts the little girl by holding the phone by its strap and pulling it away from her reach. It turns out that she had hidden her mother’s flip phone in it earlier in the day, and he had unknowingly spotted her.
Just then, Cheon-won is belted with crushed recycled cans by Women’s Association chairwoman Yang-soon (Moon Hee-kyung), who also triggers the residents’ car alarms to alert them. As Myung-sook (Jin Hee-kyung) and Dae-sung (Lee Kwang-soo) rush into the complex, they find the residents already surrounding Cheon-won. Cheon-kyu also arrives to bring Yul home, before Dae-sung could ask Yul anything.
At MS Mart, Myung-sook and Dae-sung meet with A-hee (Kim Seol-hyun from AOA) and Officer Kim (Lee Yoon-hee) to discuss the probability of Cheon-won being the culprit. However, Dae-sung’s hunch says otherwise, as Cheon-won wouldn’t have let himself be seen with Yul if he’d taken her hostage. Moreover, Dae-sung adds, Yul was barefoot, “as if she had to run away urgently”. Officer Kim abruptly takes his leave and heads straight to Cheon-won’s apartment to search for Yul’s missing shoes (despite not having a warrant), and also to warn the ex-convict that he should “live like [he doesn’t] exist”, especially after he’d let him go way back when.
Back at the supermarket, Myung-sook receives a call from Cheon-kyu, informing them that his daughter is fine, and adds that Yul had not been a witness to any situation in particular. As for the shoes, he reckons that they could have been taken by bullies from another neighbourhood. Over at Cheon-kyu’s house, Yul wakes up with a startle after calling out for an “older sister”, but doesn’t let on too much when her father asks. Cheon-kyu gently advises Yul against interacting too much with the “supermarket guy”, and tells her to let him know especially if any older man who is especially nice to her “tries to see the skin under [her] clothes”.
The next day, Saeng-seon pulls Dae-sung into the freezer and asks if Jung-yook (Lee Kyo-yeob) is on the suspect list, thinking out loud as to whether it’s safe for him to walk Ya-chae (Oh Hye-won) as he’d offered. Saeng-seon thinks back to all the times she’d caught on to Jung-yook’s frustrated looks whenever Ya-chae mentions her husband, or when he sees her flirt with a male customer. Later, Dae-sung tests the waters with Jung-yook, suggesting that he comes along when the latter walks Ya-chae home, only to be brushed off by his long-time friend. Feeling unsettled, Dae-sung heads out to follow them.
Ya-chae appears visibly uneasy to be around Jung-yook, and gets especially nervous when he asks about accompanying her to the “third-floor entrance” – without ever telling him where she lives. Jung-yook tries to talk to her, but Ya-chae wrests her wrist from his grasp, and shakily tells him not to walk her home in future. In her apartment, Ya-chae panics when her door lock code is entered externally, and runs into her room to hide. The hooded intruder then attempts to enter her room by sliding a knife blade between the door and its frame. After pushing a chest of drawers against the door, a petrified Ya-chae shakily calls the emergency hotline.
Officer Kim and A-hee – who were contacted by Dae-sung – get to Ya-chae’s apartment to investigate the intrusion and to comfort the shaken woman. When asked about her husband’s whereabouts, Ya-chae says that he’s away on business in the Philippines. Dae-sung also tries to contact Jung-yook, who ignores the call while in his room. Surrounded by other dark-themed monochrome drawings plastered on his walls, Jung-yook furiously shades his drawing of a cross-legged figure hiding in the darkness – until the pencil lead snaps.
Another day begins for Dae-sung, who meets up with A-hee early in the morning for a quick catch up near the police substation. He returns to MS Mart to see Jung-yook looking dazed, and follows him out of the building to deter him from smoking. Unbeknownst to Dae-sung, a distraught Ya-chae had quit her job moments before, and Jung-yook was stopped by Myung-sook from trying to talk with her. Before heading back in, Jung-yook asks Dae-sung if he’d happened to see the sketchbook in his locker. Dae-sung says that he hasn’t seen it, though he recalls grabbing it to show Saeng-seon as his “alibi”, after rummaging through her locker.
Meanwhile, Yang-soon does her rounds at the apartment block, and once again spots the running electrical metre of a vacant apartment under Cheon-kyu’s purview. After heading in, she discovers an industrial-sized freezer in one of the rooms. Yang-soon opens it out of curiosity and her eyes widen in shock at what she sees – just as a gloved hand covers her mouth.
As Dae-sung prepares for Saeng-seon’s welcome party on the rooftop later that evening, Yul comes into the supermarket and slips her flip phone into the lost and found box. Myung-sook greets her and hands her a chocolate egg, and Yul, in turn, asks about Ya-chae’s absence. The older woman tells the little girl that Ya-chae’s taking a break as something scary had happened to her. “Does she think she’s safe outside of the apartment?” Yul wonders out loud.
Just then, Myung-sook’s husband Young-choon (Shin Sung-woo) rolls in a toy catcher machine, having been talked into purchasing it by the neighbouring stationery store owner. When asked if she wanted to add a toy into the catcher machine, Yul takes the opportunity to hide her flip phone in a little koala pouch, and places it among the other toys.
Up at the rooftop, Saeng-seon opens up to Dae-sung and A-hee about how she’d gotten her previous criminal record: her parents reported her to the authorities for theft, and also as a way to “fix” her after she had come out as being transgender. The mood lightens up gradually when A-hee asks Saeng-seon about the make-up products she uses, which Saeng-seon is more than happy to share. Just then, part timer Al-ba (Jo A-ram) appears at the rooftop with a bag of meat from Jung-yook, saying that he was not in the mood to join the party. Dae-sung tries to contact Jung-yook, but to no avail.
At the vacant apartment, a defiant yet terrified Yang-soon pleads for her life with her captor, who holds a glue gun towards her line of sight. She later manages to escape, with glue all over her face, frantically running down the corridor, Bumping into a wall, she gets cornered by her captor, who lifts her body over the ledge. Dae-sung, who had been tasked by Myung-sook to go on a late-night delivery run at that very apartment complex, receives a call from A-hee on his cell phone – just as Yang-soon’s body plunges to the ground with a thud.
While being accompanied home by Dae-sung after the welcome party, a tipsy A-hee follows the lead of an on-duty ambulance, where they both find out what happened to Yang-soon. Gong-san, who had found her body, gives her statement to the police officers at the scene. Just then, Detective Choi (Bae Myung-jin) is handed a glue gun found on the fourth floor. When he heads up to inspect the area, he finds out that Cheon-won lives just next door.
The next morning, Dae-sung awakes from a nightmare to see a glue gun in his face and Jung-yook in his room. Shocked, he kicks the glue gun away, which leaves a cut on Jung-yook’s chin. Feeling wronged, Jung-yook yells back that he was helping Dae-sung glue down the gaps on his door when he heard him screaming. After they’d both calmed down, Dae-sung finds out from Jung-yook that the latter was drinking alone near his apartment just the night before. Separately, Ya-chae continues to fend off Jung-yook’s frequent calls.
Detective Choi comes to MS Mart to elicit a ‘confession’ from Dae-sung with claims so ridiculous, no one could take him seriously. After being told that a ‘witness’ had come forward, Dae-sung heads to the police station to prove his innocence. This turns out to be Cheon-won, who digs a hole for himself by claiming that a glue gun “with his prints” is missing – when in actual fact, he appeared to be the one who left it at the ledge Yang-soon fell from. With this new information, Dae-sung is asked to leave, while Cheon-won continues to be kept behind for questioning.
Unable to reach Ya-chae, a nervous Jung-yook attempts to leave the supermarket early, but Myung-sook makes up a story about an urgent order to be fulfilled. As the butcher gets to work, Myung-sook pulls Saeng-seon aside and asks if she could secretly follow Jung-yook. Saeng-seon, in turn, dolls herself up to look the part, much to Myung-sook’s amazement and admiration.
Saeng-seon tails Jung-yook to Hanbom Apartments – where Ya-chae lives – but loses him as her heel gets stuck in a sewer cover. Just then, Ya-chae – who was unable to meet the person she’d arranged to meet at a “crowded place” – returns to the complex with her luggage in tow. In her apartment, she has a drink of water and heads into the bathroom, when she’s suddenly pulled in. At the same time, A-hee – informed by Myung-sook about the sting operation – hears a shriek after she pulls into the parking lot. She runs into the apartment block, with Saeng-seon following behind her.
Jung-yook, however, is already outside the apartment, and uses a tool to pry open the steel window bars. He climbs through the window and witnesses a man – later identified as Ya-chae’s ex-husband, Oh Myung-soo – dragging her by the hair into the kitchen and continuing to beat her. Jung-yook manages to free her and starts to beat the ex-husband up in a blind rage, before A-hee manages to intervene. Myung-soo is later arrested for stalking, assault and sexual violence; when asked by Saeng-seon if he was also involved in her friend Kyung-a’s murder, his alibi of only recently returning from the Philippines checks out.
On his way back to the supermarket, Dae-sung spots Gong-san dashing across the road, and decides to follow her. He eventually sees her beating up a cowering young man in an alleyway and leaving; the young man too hurriedly gets up to leave when Dae-sung approaches him. Dae-sung tails Gong-san all the way to Unhaeng Hospital, where Yang-soon has been warded after her harrowing brush with death.
At the hospital, Gong-san manages to sweet-talk her way into seeing Yang-soon, who’s heavily bandaged and still in a coma – or so she’d been told by the police officers on guard outside her door. In the room, Gong-san pulls out a bottle of pine-scented bleach, uncaps it and puts the bottle cap to Yang-soon’s nose, telling her that this was what she’d smelt like when she came across her body. As Yang-soon appears to react to the smell, Dae-sung and a police officer barge into the room, and ask Gong-san to leave.
Later, as Dae-sung approaches a nurse to ask about a non-existent “friend”, Gong-san interrupts by asking about a Vietnamese patient called Ngyuen Thi Zhonglin, who’s supposedly warded at the hospital. The nurse was unable to provide any information, but Gong-san finds out from a volunteer caregiver she’s acquainted with that she has not seen the woman, who turns out to be Yul’s mother and Cheon-kyu’s wife.
Dae-sung is shocked when he overhears them both talking about how Yul’s mother had been frequently beaten, and Gong-san tells him matter of factly that “it’s common for brides from overseas to get beaten, they just don’t talk about it”. She even mentions how Cheon-kyu would buy concealer for his wife, to “cover up the cuts and bruises”.
Back at the mart, Dae-sung finds out from his father that he’d discovered blisters on Yul’s arms, hidden by her sleeves. He decides to personally bring Yul the items she’d left behind, and she approaches him outside her father’s closed real estate office. Yul asks Dae-sung about what the police had said to him, and he tells her that they were mistaken about him being a suspect. He does a double take when the little girl says, “I know you didn’t do it”, and crouches to her level to ask how she knows. He discovers fresh blisters and bruises on her arm, and tries to get her to tell him if her father had caused them – just as Cheon-kyu approaches them both.