Park Chang-ho is an underachieving lawyer with a dismal 10% success rate, mockingly nicknamed 'Big Mouth' by his peers due to his tendency to talk big without backing it up. His life takes a harrowing turn when he is hired to investigate a high-profile murder case, only to be framed as the legendary con artist known as 'Big Mouse.' To survive and protect his family, he must shed his incompetent persona and dive deep into the dark, corrupt underbelly of the elite to uncover the truth. As he navigates the dangers of prison life and the machinations of powerful figures, his wife, Go Mi-ho, a dedicated and sharp-witted nurse, begins her own investigation to clear his name. Together, they face systemic corruption, life-threatening conspiracies, and the constant pressure of being hunted by both the law and the criminal underworld. The series is a high-stakes noir thriller that explores themes of justice, survival, and the transformation of a man pushed to his absolute limits.