Set in the 19th-century Joseon Dynasty, the story follows Park Yoon-kang, the son of the greatest swordsman of the era. After his father is framed for treason and his younger sister is sold into slavery, Yoon-kang is forced to flee. He eventually returns to Joseon disguised as a Japanese merchant, wielding a Western-style rifle to seek revenge against those who destroyed his family. As he navigates a rapidly changing society caught between tradition and modernization, Yoon-kang becomes a hero to the common people, known as the 'Gunman.' Alongside his love interest, Jung Soo-in, he finds himself embroiled in political conspiracies and the struggle for power, ultimately fighting to protect the people and bring justice to a corrupt government.