"Underdog" follows three boxers who are all underdogs. Suenaga Akira is a boxer, who struggles to succeed as a boxer. Omura Ryuta is a young boxer, who grew up in an orphanage. Miyagi Shun challenges a boxer, following the plans of a TV program.
16-year-old Haruka (Ai Hashimoto) lost the people she loved the most in a fire. Haruka is also physically and emotionally wounded from the fire, but she still dreams of becoming a pianist. She takes part in an upcoming competition. Inauspicious incidents then occur around her. Musical student Yosuke (Shinya Kiyozuka), who teaches piano to Haruka, tries to solve the case.