Do Min-joon (Kim Soo-hyun) is an alien that landed on Earth in the year 1609, during the Joseon Dynasty. As he saves 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 human appearance, enhanced physical abilities involving his vision, hearing and speed, and a cynical, jaded view regarding human beings. As he never ages, Min-joon is forced to take on a new identity every ten years, having been a doctor, astronomer, lawyer, and banker, and now presently working as a college professor.
Cheon Song-yi (Jun Ji-hyun) is a famous Hallyu actress, having attained stardom since middle school, however her haughty demeanor has earned derision in the entertainment industry and social media. Furthermore her spendthrift mother has mismanaged her finances, while younger brother Cheon Yoon-jae (Ahn Jae-hyun) is estranged by her success. Lee Hee-kyung (Park Hae-jin) has been Song-yi's friend since middle school and remains in love with her while being continually rejected. In turn, Yoo Se-mi (Yoo In-na), who is frequently cast as a supporting actress alongside her childhood best friend Song-yi, has had a crush on Hee-kyung since middle school despite her love being unrequited. As a result Se-mi secretly harbors a deep jealousy towards Song-yi for standing in the way of her career and love interest.
With only three months left before Min-joon's long-awaited departure back to his planet of origin, Song-yi suddenly becomes his next-door neighbor in the condominium. 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 that she at a young age resembles Yi-hwa whom he fell in love with 400 years ago in the Joseon era. Min-joon and Song-yi eventually fall in love; Min-joon aims to leave Earth without being emotionally attached he tries but fails to keep himself away from her, while Song-yi initially is not understanding of his impending departure but ultimately accepts letting him go to assure his survival.
Song-yi's career goes into a downturn as she is dropped by her talent agency and sponsors, due to a backlash against her recent behavior, particularly exacerbated by rumors that she caused the suicide of her arch-rival, actress Han Yoo-ra. Earlier at a celebrity wedding Song-yi had witnessed Yoo-ra in a secret relationship with Lee Jae-kyung (Shin Sung-rok), the elder brother of Hee-hyung. Consequently Jae-kyung tries to silence Song-yi until Min-joon brokers a deal to spare her in return for burying the evidence. But Jae-kyung turns out to be much more dangerous than Min-joon suspected, learning to exploit Min-joon's weaknesses, and even injuring Se-mi's older brother, a prosecutor investigating Yoo-ra's suicide. Min-joon, despite being discrete in using his special abilities eventually get the attention of police, while also losing control of his powers as his departure date nears. While jealous of Min-joon for winning Song-yi's heart, Hee-kyung nonetheless works together with him to protect her from Jae-kyung.
