Punch is a record of the last six months of Park Jung-hwan's life. He is the chief of the anti-corruption investigation team for the Supreme Prosecutors' Office. To get to his position, Jung-hwan has made compromises to achieve what he thought of as the greater good, though it meant losing some of his soul in the process. But when he gets diagnosed with a malignant brain tumor and told that he only has six months left to live, it makes Jung-hwan reexamine his life choices. He decides to pursue justice whatever the cost, even if it means sacrificing his life. This is his last attempt to make things right, one final "punch" against the crooked world. And his main goal is bringing down his boss Prosecutor General Lee Tae-joon, whose friendly public face masks his unscrupulous morals and rampant corruption.
Helping Jung-hwan in his quest is his ex-wife, Shin Ha-kyung. Ha-kyung is an idealistic prosecutor for the Seoul District, and chose her profession over having a lucrative law career. She divorced Jung-hwan because he was obsessed with ambition and never had time for her and their young daughter, Ye-rin. But that doesn't mean she doesn't still care for him, though her concern is mixed with resentment.