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. Cheon Song-yi (Jun Ji-hyun) is a famous Hallyu actor who attained stardom as a schoolchild; her haughty demeanor has earned derision in the entertainment industry and on social media. Song-yi's spendthrift mother has mismanaged her finances, and her 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 but is continually rejected. In turn, Yoo Se-mi (Yoo In-na), Song-yi's childhood friend, who is frequently cast in a supporting role alongside 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 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 that 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. Song-yi's career goes into a downturn when her talent agency and sponsors drop her in a backlash against her recent behavior, particularly rumors that she caused the suicide of her arch-rival, actor Han Yoo-ra. Earlier at a celebrity wedding, Song-yi had discovered Yoo-ra was in a secret relationship with Lee Jae-kyung (Shin Sung-rok), the elder brother of Hee-hyung. Jae-kyung tries to silence Song-yi until Min-joon brokers a deal to spare her in return for burying the evidence. Jae-kyung, however, turns out to be much more dangerous than Min-joon suspected, learning to exploit Min-joon's weaknesses and injuring Se-mi's older brother, a prosecutor who is investigating Yoo-ra's suicide. Min-joon, despite being discreet in the use of his special abilities, eventually draws the attention of police while losing control of his powers as his departure date nears. While jealous of Min-joon for winning Song-yi's heart, Hee-kyung works with Min-joon to protect her from Jae-kyung.
