Do Min-joon (Kim Soo-hyun) is an alien who landed on Earth in 1609 during the Joseon Dynasty. He saves a girl named Seo Yi-hwa from falling off a cliff and misses his return trip to his home planet and is stranded on Earth for the next four centuries. He has a near-perfect human appearance, enhanced physical abilities involving his vision, hearing and speed, and a cynical, jaded view of human beings. Min-joon never ages and is forced to take on a new identity every ten years; he has worked as a doctor, an astronomer, a lawyer, and a banker, and is now working as a college professor.
With only three months left before Min-joon's long-awaited departure to his planet of origin, Song-yi suddenly becomes his next-door neighbor in the condominium where he lives. Slowly, Min-joon finds himself entangled in Song-yi's crazy and unpredictable situations, saving her multiple times using his special powers and eventually acting as her manager due to his vast legal knowledge. He finds out she at a young age resembles Yi-hwa, with whom he fell in love with 400 years earlier. Min-joon and Song-yi eventually fall in love; Min-joon aims to leave Earth without being emotionally attached so he tries to avoid her but fails. While Song-yi initially does not understand his impending departure, she ultimately accepts letting him go to assure his survival.