Undercover Miss Hong's third episode commences with Keum-bo fleeing the office, her heart racing as she dashes into the bustling streets, fearing Jung-woo's relentless pursuit. Jung-woo exits his car, his eyes fixed on the elusive figure of Keum-bo, who, in her disguise, declares herself to be Jang-mi. With a swift turn, she disappears into the night, leaving Jung-woo alone in the street, lost in thought as he recalls gifting her with matching sand timers during their past romance.
That night, Jung-woo is seated in his apartment, his mind wandering back to their shared memories. He reaches out to a colleague to inquire about Keum-bo's whereabouts, his curiosity piqued by the mystery of her disappearance. Meanwhile, Keum-bo returns to their dormitory, where her roommates are gathered, eager for a late-night snack.
Bok-hee teaches No-ra, Keum-bo, and Mi-sook the art of sneaking food into their dorm room through the window. Despite her suspicions about Bok-hee's true intentions, Keum-bo accepts a soft toy keychain as a token of gratitude from her roommate.
In their room, Keum-bo discovers the slush fund ledger and is about to open it when Bok-hee enters, sparking an argument that draws the attention of Warden Kim. As the girls hastily hide their contraband, Kim interrogates them about the commotion. Keum-bo plays the victim, accusing Bok-hee of calling her a thief. When Bok-hee's purse is searched and found to be empty, Kim tries to mediate between the two girls.
The argument continues over Chuseok holiday lunch, with Keum-bo storming out of her room to sleep on the living room couch. The next day, Jung-il finds Keum-bo in the break room, making coffee and taunting her. Keum-bo returns his snide remarks with a request for him to behave like an adult.
Later that day, Bok-hee and No-ra eavesdrop on the secretaries gossiping about Keum-bo and Jung-il's pursuit. They try to warn Keum-bo but are met with resistance. In retaliation, Bok-hee uses Jung-il's toothbrush to clean his dusty computer while No-ra replaces his vitamins with laxatives. The episode ends with tension brewing between the roommates and Keum-bo's growing suspicion about Bok-hee's true motives.
Keum-bo, fueled by courage, steals a receipt that Jung-il intends to use for his expense reports. The next day, Jung-il arrives at work, takes his vitamins, which happen to be a laxative. Simultaneously, Keum-bo is surrounded by her Risk Management teammates, working tirelessly to develop a new information-sharing system. Yong-gi leads the meeting and enthusiastically presents his idea to the team. Albert and Jin-mok seem unsure of how to proceed, but Keum-bo is confident that Yong-gi can create the system within three days. Yong-gi is touched by Keum-bo's faith in him and begins to develop feelings for her. He works diligently throughout the day with Keum-bo's support, while Albert and Jin-mok pass their time watching movies.
Albert notices the strong bond between Yong-gi and Keum-bo and becomes jealous. Days pass, and Yong-gi becomes so engrossed in his work at his desk that he hardly moves. Albert attempts to take credit for Yong-gi's work, but Keum-bo insists that it was all thanks to Yong-gi's hard work.
The next day, Albert tells Jung-woo about the software and how Keum-bo (Jang-mi) and Yong-gi worked tirelessly on it. Jung-woo praises Albert for leading his team well. Later that day, Yong-gi helps the employees create their new accounts to access the software. The interns and secretaries discuss how the software is helping them reduce the number of document copies they need to make.
At an executive meeting, Deok-gyu informs Chairman Kang of Albert's team's successful creation of the software for the company. After the meeting, Albert informs his team that they do not need to work for the rest of the year due to the success of the software.
Keum-bo decides to focus her efforts on uncovering the identity of Yeephee. In private conversation with Yong-gi, she learns that Gyeong-dong owns a dog named Yeephee. She endeavors to follow the man and presents herself at the trading department to hand over a file. There, she assists No-ra, who is busy preparing coffee and lunches for the traders. While trying to help No-ra, Keum-bo attempts to rummage through Gyeong-dong's desk and files. However, she discovers a locked desk and is unable to gain access. She informs Jae-beom about her findings, who provides her with a lockpicker tool. No-ra informs Keum-bo that Gyeong-dong rarely leaves his desk unless it's for brief trips to the bathroom.
The next day, Keum-bo offers to help No-ra bring lunch to the traders. Meanwhile, at the Risk Management office, Yong-gi appears with a fresh perm and brings cookies for Keum-bo. Albert shows up with freshly highlighted hair to impress Keum-bo. Jin-mok teases the two for chasing after an intern.
Keum-bo notices a trader attempting to engage in insider trading and tries to stop him. She causes a commotion when she halts the guy from making a fake sale and informs the stock exchange about it. Chairman Kang and Jung-woo meet with Deok-gyu and Gyeong-dong to discuss this trading blunder.
Ju-ran reveals that it was Keum-bo (Jang-mi) who stopped the trading. The incident becomes company-wide news, and the trader is investigated by the Financial Supervisory Service. That night, Jae-beom gives Keum-bo a piece of his mind for drawing attention to herself.
Keum-bo claims that she couldn't let the illegal activity slide, considering she's an employee of the supervisory service. He tells her about the online chat group, "The Yeouido Pirates," which posted about the incident.
Jae-beom wonders who posted the news on the chat group and if a Hanmin employee was an admin of the online community. He reveals that the chat group is invite-only and that every member gets one free invitation to join. That night, Keum-bo tries to get her roommates to invite her to the chat group. Mi-sook reveals that she gave her invite to someone else, while Bok-hee invited No-ra, and No-ra invited Mi-sook to the group. Keum-bo is dejected after being left out and tries to secure an invite from her team members. Yong-gi reveals that he has used up his invite. Albert offers to give his invite to Keum-bo and helps her create an account. Keum-bo creates the account and finally enters the chat group. She browses through all the posts and tries to post something about herself to figure out who the mole is.
The following day, Ju-ran cautions Keum-bo (Jang-mi) against discussing the trading mishap with the media, promptly informing her about the impending investigation later in the day. Jae-beom and Dong-gi arrive at Hanmin Investment & Securities to probe into the matter. Dong-gi requests a meeting with Jang-mi, the intern who halted the trading, while Jung-woo instructs Bok-hee to summon Keum-bo to the investigation room. Jae-beom attempts to thwart this, but when all other attempts fail, he feigns a heart attack. Keum-bo is rushed to the hospital before she can reach the investigation room.
That evening, Keum-bo thanks Jae-beom for his assistance. They discuss how Hanmin Investment & Securities were involved in whatever the trader was doing. Jae-beom reveals that the company had bribed higher-ups at the Financial Supervisory Service and adds that there was nothing they could do to incriminate them for insider trading.
The next day, Keum-bo (Jang-mi) is summoned for a disciplinary hearing. Albert defends Keum-bo before Chairman Kang and Jung-woo, who claims using Keum-bo as a scapegoat is their best option. Keum-bo is upset about being used as a scapegoat despite doing the right thing. Albert encourages Keum-bo and urges her to defend herself at the hearing.
Meanwhile, Jung-woo's colleague informs him of Keum-bo's past involvement with Hanmin Investment & Securities and her demotion after Myeong-hwi's death. After the call, Jung-woo attends the disciplinary hearing where he confronts Keum-bo.
A flashback reveals Keum-bo being fired from her job as a public accountant after she reported errors with Hanmin Investment & Securities. Despite being her boyfriend and a senior, Jung-woo signs the report to have Keum-bo fired. Keum-bo's teammate ends his life after being used as a scapegoat for Hanmin. Jung-woo begs Keum-bo to forget what happened, but she decides to end their relationship because he refuses to do what is right.
The episode concludes with Keum-bo throwing away the ring that Jung-woo had given her. Present-day Keum-bo (as Jang-mi) sits across from Jung-woo at the disciplinary hearing.
