Do Min Joon first arrived on earth in 1609, but in order to protect a girl he met, he missed his chance to return to his home planet. Four centuries later, he is still living in Korea and working as a professor, the latest of many identities he's taken on through the years. Understanding that he doesn't belong to this world, the exceedingly rational Do Min Joon maintains distance from everyone save for one friend Jang Young Mok who knows his secret. All is quiet and well for him until top actress Chun Song Yi moves in next door and comes barging into his cloistered life.
Loud, proud and bumbling yet irrepressibly lovable, Song Yi throws Do Min Joon off guard, and he can't help but protect her when her career takes a stumble. The two begin to fall in love despite themselves but Do Min Joon's days on earth are numbered.