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 fall madly in love after finding out the loneliness they share equally. But when the ex-fiancé of Sun-jae, Min-joo, reappears in Sung-jae's life, they face conflict.