In Episode 5 of Concrete Market, Cheol-min discovers the connection between Hee-ro and Se-jeong. A lackey of his learned from a fellow resident that Se-jeong had a younger sister who had mysteriously disappeared one day. The little sister bore a striking resemblance to Se-jeong. Upon learning that Hee-ro began staying in Se-jeong's room shortly after her death, Cheol-min realizes that she knew Se-jeong and was now protecting her sister. He shows up at Hee-ro's room to propose a deal. He knows that Hee-ro desires revenge and protection for Se-hee. He suggests that Hee-ro help him take over Chairman Park's room, and in return, he will not reveal Hee-ro's motive and Se-hee's existence.
The water shortage persists, and Chairman Park kills some residents who try to escape to maintain control. Cheol-min's threat pushes Hee-ro into a corner, prompting her to brainstorm a way out. She decides that the first step should be to get rid of Tae-jin, who is a hindrance to her plans.
Hee-ro takes advantage of the water shortage to draw her next plan. Due to the shortage, the water is rationed to one bottle per day. She convinces Tae-jin to agree to buy water from the residents at a higher price. She sells him a plan to use the water to make a deal with Cheol-min. Since his team needs more water to keep the alcohol business running, they can offer to exchange it for Mi-seon's freedom. Tae-jin sets the plan in motion without realizing that Hee-ro is working with Cheol-min.
Next, Hee-ro poisons the water supplied by Chairman Kim and uses Cheol-min's lackeys to spread the news that someone wearing a ghost mask sneaked into the water storage area. He insists that, although the person was wearing a ghost mask, they were also wearing regular clothes, suggesting that a resident deliberately poisoned the water.
Meanwhile, Mi-seon uses the water poisoning as an opportunity to become the Chairman's woman. She uses her knowledge of chemicals, gained from her work in that field before the earthquake, to disinfect the water and earns the Chairman's favor.
Then, Hee-ro snitches to Chairman Park, using the deal to exchange information for medicine. She brings him the mask, saying that she got it from room 209, which is Tae-jin's room.
Chairman Park is reluctant to believe that Tae-jin has betrayed him. Hee-ro, with a sense of urgency, explains that Tae-jin was hoarding water and intentionally poisoning it to profit by selling it back to the residents at exorbitant prices. Chairman Park seems to believe the story and hands over the medicine to Hee-ro.
Tae-jin, driven by a desire to free Mi-seon, goes to room 802 to inform her about the deal. He finds Cheol-min waiting for him, ready to break the news that their deal has fallen through because Mi-seon is now Chairman Park's woman. Tae-jin is devastated and wonders why Mi-seon couldn't have waited just a little longer for him to get her out. Mi-seon shows Tae-jin a notebook, and whatever he sees convinces him that Chairman Park is an evil man.
On his way out, he runs into Chairman Park, who is in a trance after the conversation with Mi-seon. When asked about the water, Tae-jin doesn't argue. The chairman slaps him, and Tae-jin simply apologizes and keeps walking.
In his room, Chairman Park is highly skeptical that Tae-jin would be so reckless as to poison the water and go against him in the market. It turns out that he has more informants, and one is from Cheol-min's camp. He knocks on the pharmacy window and hands over the information.
The informant is the deaf-mute member of Cheol-min's group. He was in the room earlier when Hee-ro and Cheol-min were making plans to usurp Chairman Park's throne. Hee-ro had suggested that Cheol-min ask him to leave the room, confident that he wouldn't hear them or read their lips. This was an oversight on Cheol-min's side because he could read lips, but surprisingly, he didn't snitch on them. He insisted that Tae-jin is depressed and acting recklessly.
Chairman Park still feels something is off. He thinks back to his interactions with Hee-ro and decides to test her again. At the end of the episode, he calls Hee-ro to his room and informs her that he wants to kill Tae-jin.
