“Big Mouth” Lee Jong-seok avenged Kim Joo-heon by killing him with radioactive-contaminated water.
When Park Chang-ho (Lee Jong-seok) was having discussions about candidates for the Mayor position, Ko Mi-ho pointed out the fish farm, raising questions about its relationship with NK Chemical. Although someone insisted that the water used in the fish farm was just groundwater, Ko Mi-ho confessed that she has suffered painful symptoms due to that water. Ko Mi-ho then confirmed that she used contaminated water at the fish farm.
Ko Mo-ho joined the citizen panelists for a live broadcast and planned to release a video. Choi Do-ha (Kim Joo-heon) appointed a Bitcoin fraudster as a citizen panelist to attack Park Chang-ho. Choi Do-ha said, “You said you would resign if the allegations of virtual assets turned out to be true. I think this is the last chance for candidate Park to keep his minimum promise to the people”, urging Park Chang-ho to resign. Park Chang-ho responded, “The truth will be revealed and it is the people who will judge it.”
Park Chang-ho was surprised by the appearance of Ko Mi-ho. As Choi Do-ha said Ko Mi-ho was not eligible as a panelist because she’s Park Chang-ho’s wife, Ko Mi-ho said, “I came out as a radioactive victim, not Park Chang-ho’s wife”, adding “I was recently diagnosed with the end-stage of Lymphoma”. Since Park Chang-ho stared at Ko Mi-ho as he didn’t know anything about her illness, and Ko Mi-ho tried to smile while crying. When the video was released, Choi Do-ha said, “If that video is real, I want to request an investigation by the prosecution without sanctuary”.
However, the prosecution failed to identify any clear allegations, and Choi Do-ha was re-elected. Park Chang-ho was drinking consolation wine in his office, and a mysterious box arrived in front of Ko Mi-ho. Inside the box was a note from Hyun Ju-hee (Ok Ja-yeon). What Hyun Ju-hee sent was Choi Do-ha’s cannon phone. Park Chang-ho asked Hyun Ju-hee to testify that the cannon phone belonged to Choi Do-ha. Hyun Ju-hee agonized then said she would testify.
Ko Mi-ho attended Choi Do-ha’s trial although she grew weaker. Hyun Ju-hee, who decided to testify, did not arrive, and Choi Do-ha made director Cha stand as the witness to claim to have used the cannon phone. Director Cha showed that he continued to use what Chairman Kang had been using, and that he contacted Choi Do-ha using that phone. Hyun Ju-hee was locked up. At the trial, she was acquitted due to insufficient evidence.
Choi Do-ha showed his mean personality by saying “Stay healthy” to Ko Mi-ho, who asked about Hyun Ju-hee’s whereabouts. Ko Mi-ho collapsed from shock. She was hospitalized but eventually died. Park Chang-ho held Ko Mi-ho’s hand tightly and said, “I’m sorry. I was happy.”
Choi Do-ha vomited blood after swimming. Park Chang-ho said, “Many people died because of your radioactive contaminated water. You should suffer the same.” The water in the swimming pool where Choi Do-ha dived was radioactively contaminated. As a result, Park Chang-ho’s revenge was successful, but Ko Mi-ho had already passed away. Ko Mi-ho asked Park Chang-ho to become a good Big Mouse. Park Chang-ho remembered Ko Mi-ho’s words.