The series Pro Bono commences with the tale of deceit and abuse perpetrated by the powerful CEO Jang, but the public believes that Judge Kang Da-wit, a publicly anti-corrupt judge, will make the right call. Renowned for his friendliness, generosity, kindness, confidence, and popularity, he is affectionately known as the People's Judge. Despite his nickname, he remains unswayed and metes out a 10-year sentence and a 70 billion won fine to CEO Jang. As expected, his public rating skyrockets.
In his free time, Da-wit indulges in K-pop dancing and manga reading, pretending to be humble while secretly craving the fame that comes with being appointed a Supreme Court Justice. Public opinion matters, but it's not enough for him. He grovels and drinks with corrupt Senior Chief Justices, who constantly taunt him. In the court office, he offers help to a young girl struggling with paperwork, only to be rude when he realizes she's a public interest attorney and not a court employee who can vote for him.
When Da-wit is nominated as a Supreme Court Justice, he visits his mother's grave next to an apricot tree. A flashback shows her working in a factory with back pain, but she always kept a strong face in front of baby Da-wit. The only thing he could give her were local, sour apricots, but she appreciated the thought. She eventually dies from overexertion in the years to come.
An old elementary school friend, Kim Ju-seop, suddenly appears and takes Da-wit's autograph while getting him drunk. The next day, Da-wit finds 1.2 billion won in his car, realizing it's a trap but the damage is done. There's dashcam footage of an extremely drunk Da-wit accepting the box of money from Ju-seop. Da-wit's boss, Justice Shin, is furious as "Ju-seop" is actually a convicted serial conman called Yoo Jae-beom. The real Ju-seop died 10 years ago.
To cover up the video and bribery rumors, Da-wit is barred from practicing law. He is upset as his mother's dying wish was that he be successful so that people wouldn't look down on him, like they did to her.
He harbors a deep-seated belief that his savior is Oh Jung-in, a young and charming colleague who once had a crush on him. She is the daughter of Chairman Oh, who successfully lifts the ban on Da-wit's career. As a gesture of goodwill, she extends an invitation to join Oh & Partner, one of the most prestigious law firms in the country.
On his first day, Da-wit attempts to play the role of the passionate and friendly colleague. However, his smile quickly fades as he is led to the pro bono division - a cramped and dimly lit office in the basement with no natural light. He is introduced to three lawyers and their furry clients, the dogs they are defending. The fourth lawyer, Park Gi-ppeum, is the same girl he was rude to in court.
Da-wit is devastated to find himself demoted to a "free" lawyer and takes refuge in the garden. Suddenly, one of the dog clients jumps on him, causing him to lament his unfortunate turn of events.
