Kang Poong-ho is a pest exterminator by day and a K-1/mixed martial arts fighter by night. He is a single father raising his 7-year-old son Kang San after the boy's mother, his first love Yoon So-yi abandoned them to pursue her ambition of becoming a pianist. Despite life's trials, Poong-ho never loses his optimism and sense of humor. He meets and falls in love with Jeon Ha-ri, a bright and bubbly medical student from a wealthy family. But an obstacle to their romance arises when they learn that Ha-ri's soon-to-be stepmother is none other So-yi, and that it was Ha-ri's surgeon father Jeon Ki-suk who sponsored her piano studies abroad. As So-yi re-enters their lives, San is diagnosed with a brain tumor, and Poong-ho is willing to go to any lengths to save his beloved son.