South of the Sun is a love story between Kang Sung Jae, who has been in prison despite his innocence and cut off from the world, and Jung Yeun Hee, who appears to be happy on the surface but is suffering from isolation. They find each other through a letter by coincidence and madly falls in love after finding out the loneliness they share equally. But when ex-fiancee of Sun Jae, Min Joo, reappears in Sung Jae's life, they face conflict.