Episode 4 recap: Pang Hongmei admitted her mistake.

Ji Danyang asked Pang Hongmei if the issue with the brass sheet had been resolved, and Pang Hongmei replied that it was a pencil bomb. Ji Danyang was also surprised, as it seemed unlikely that a pencil could be turned into a bomb. The missing five kilograms of explosives have not yet been found. Zhang Yan stated that the enemy agent has been identified as Zhang Ashui, who was injured by a gunshot and needs medical treatment. However, their unit has not received any news about gunshot victims at the hospital.

Due to the rumors circulating outside, Bai Hui was hesitant to directly hand the medicine to Ji Danyang and instead placed it in his lunchbox. When Ji Danyang came over to have lunch, Pang Hongmei happened to be present as well. Ji Danyang didn't notice anything in the lunchbox, so Pang Hongmei quickly asked him to step back and opened it herself, discovering a package of medicine. Pang Hongmei noticed that Bai Hui was watching this exchange suspiciously and approached her to ask if she was the one who provided the medicine.

Bai Hui admitted it, and Pang Hongmei stated that it needed to be tested. The team leader came over and informed Pang Hongmei that the medicine had been examined and there were no issues. Pang Hongmei also had a scar removal ointment, which was also deemed safe. Pang Hongmei couldn't understand why Bai Hui couldn't openly give the medicine, but the team leader explained that given the current situation, Bai Hui's caution was understandable.

Pang Hongmei and Ji Danyang went to Bai Hui's home, where Pang Hongmei apologized to Bai Hui, admitting that she hadn't considered Bai Hui's circumstances. Bai Hui also acknowledged her mistake, realizing that if the medicine were tampered with, her good intentions could turn into something harmful. Pang Hongmei and Ji Danyang came to apologize to Bai Hui at her home, expressing their oversight in not considering her situation.

Ji Danyang was about to leave the factory, and Pang Hongmei accompanied him. They arrived at the library, where the librarian approached them and explained that the requested materials were not available there. The librarian, aware that Ji Danyang was an expert returning to the country, offered to contact the libraries at Tsinghua University and Peking University to obtain the materials. Pang Hongmei realized that even the librarian knew about Ji Danyang, indicating how dangerous his situation had become.

While at the library, Ji Danyang coincidentally encountered Bai Hui, who suggested taking them for a walk. Pang Hongmei noticed Qian Dayong and instructed Bai Hui to keep an eye on Ji Danyang while she went to catch up with Qian Dayong.

Bai Hui and Ji Danyang arrived, and Pang Hongmei was very angry. Ji Danyang said not to blame Bai Hui, as it was his own insistence to follow along. Unexpectedly, Qian Dayong was hiding in the soy sauce jar and came out to attack Ji Danyang, but Pang Hongmei helped block him for a moment.

The initial plan to capture Qian Dayong was arranged by the team leader, but they still hadn't caught him. The team leader apologized to Pang Hongmei and Zhang Yan. Pang Hongmei took responsibility for her own mistakes and told the team leader that he didn't need to take the blame for her. The team leader explained that he was not taking the blame for her but acting as a leader. The team leader asked if Pang Hongmei knew where she had gone wrong and what they would do if this whole operation was a diversion to target Ji Danyang.

Pang Hongmei said she understood. Currently, the expert was about to arrive, and there was tight surveillance at the station. Pang Hongmei mentioned what Bai Hui's father had said before about the instrument factory being a former Kuomintang military factory, and the team leader suspected that there might be some hidden agenda by the intelligence agents. Pang Hongmei thought of the library's blueprints and checked them, discovering that one page was torn off, and it was the floor plan of the expert's building.

The expert had arrived and was arranged to wait in the cafeteria, while the expert's building still needed to be investigated. Pang Hongmei believed that if the explosives were placed in the expert's building, they should focus on the most likely areas. Pang Hongmei brought the blueprints to find Ji Danyang, who looked at them and believed they should pay close attention to the load-bearing wall. However, upon inspection, no bomb was found there.

After the expert's arrival, there would be a swearing-in ceremony in the auditorium. Pang Hongmei felt that there might be a problem in the auditorium, but Ji Danyang believed that solving the case couldn't rely on intuition alone. Pang Hongmei argued that intuition was also a form of judgment. The explosives team was missing a member named Ma San, so the team leader instructed someone to find him quickly.