In the early 17th century, UFOs appeared over North Korea, which opened up a legendary fate that lasted for four hundred years. Now, the alien who came to Earth at that time lives among ordinary people in South Korea as a university professor, Do Min-joon (Kim Soo-hyun). In three months, a comet will approach the Earth, and the alien who has been stranded on Earth for four hundred years will be able to return to his homeland. However, before that, he always thinks of the girl Xu Yihua (Kim Hyun-soo) he met when he first came, and he is eager to see a little girl he saved 12 years ago. Min-joon lives next door to the much-criticized national idol Cheon Song-yi (Jeon Ji-hyun), who is a student at his university. The initial communication was not smooth, but Min-joon soon discovered that the girl had such a deep connection with him. After thousands of years, their fate has never been broken. After several reincarnations, the man and the woman met again...