The Framing Setup: Mayor Choi Do-ha hired Park Chang-ho specifically because he was an incompetent lawyer ('Big Mouth') who could be easily manipulated. Choi Do-ha planted gold, drugs, and cash inside Chang-ho's law office walls to turn him into the fall guy for Big Mouse.
The Antagonist’s Real Goal: Choi Do-ha is not just a corrupt politician; he is seeking political dominion and revenge against Chairman Kang. Choi Do-ha murdered Professor Seo because Seo's research paper exposed the carcinogenic nature of NK Chemical's NF9 compound, which Choi Do-ha was illegally using and dumping.
The Real Big Mouse Identity: Room Leader No Park, the tarot card reader in Gucheon Prison, is revealed as the original Big Mouse. His motivation was finding out who murdered his daughter, an NK Chemical researcher who tried to leak the NF9 paper.
The Succession: After No Park is assassinated via a bomb explosion, Chang-ho officially succeeds him as the new leader of the syndicate, using Big Mouse's immense wealth and covert military power to battle Choi Do-ha.
The Finale & Fate of Characters:
- Mi-ho is exposed to NK Chemical’s radioactive wastewater while infiltrating an illegal fish farm, developing Stage 4 Lymphoma.
- She heroically testifies during a televised mayoral debate, exposing Choi Do-ha's environmental crimes before passing away peacefully in Chang-ho's arms.
- Choi Do-ha uses money and media control to win reelection and get acquitted in court.
- Knowing Choi Do-ha will never face legal punishment, Chang-ho buys Choi Do-ha's private luxury spa building and replaces his swimming pool water with NK Chemical's radioactive wastewater. Choi Do-ha swims in it, coughs up blood, and dies.
- Chang-ho continues operating as Big Mouse from behind the scenes, using illicit funds to secretly finance charities, legal aid, and public welfare.