Do Min-joon (Kim Soo-hyun) is an alien that landed on Earth in the year 1609, during the Joseon Dynasty. As he tries to help a young girl named Yi-hwa from falling off a cliff, he misses his trip back to his planet and is stranded on Earth for the next four centuries. He possesses a near-perfect appearance, enhanced physical abilities involving his vision, hearing and speed, and a cynical, jaded view regarding human beings. As time goes by, Min-joon is forced to take on a new identity every ten years, as his human appearance never ages.
In the present, he works as a college professor. With only three months left before his long-awaited departure back to space, he meets Cheon Song-yi (Jun Ji-hyun), a famous Hallyu actress who suddenly becomes his next-door neighbor. Slowly, he finds himself entangled in Song-yi’s crazy and unpredictable situations, and finds out that she resembles the young girl he fell in love with 300 years ago in the Joseon era. Min-joon tries to keep himself away from Song-yi as he needs to leave the planet, but often finds himself failing to do so.
Song-yi gets entangled in the dangerous games of Lee Jae-kyung (Shin Sung-rok), the elder brother of Lee Hee-kyung (Park Hae-jin), who has been her friend since middle school and is love with her. Yoo Se-mi (Yoo In-na) is Song-yi’s childhood best friend who has had a crush on Hee-kyung since middle school. Realizing her love is unrequited, Se-mi ultimately gives up on Hee-kyung and starts to focus on her career as an actress. As Jae-kyung tries to silence Song-yi, Min-joon finds himself saving her multiple times, and eventually, the neighbors fall for each other. But Jae-kyung turns out to be much more dangerous than Min-joon suspected, as he hurts Se-mi’s older brother, who is investigating the murder of an actress in which Jae-kyung is actively involved, and he also mysteriously starts to lose control of his powers.