The show revolves around the main character Cheon Song-yi, a Hallyu top actress who fell in love with her alien neighbor and professor, Do Min-joon. Do-Min-joon landed on Earth during the Joseon dynasty. He possesses various powers such as teleportation, pausing time, telekinesis, bionic ears, electromagnetic field and random vision of the future. He saved a poor 16 year old widow girl’s life once, he missed his flight back to his planet, she fell in love with the widow girl but then she died soon after. Do Min-joon can’t do anything but to wait for the next flight back home. While patiently waiting, he spent his 400 years living different lives and professions under different names. Fast forward to 2002. Do-Min-joon had a vision of a girl who looks like the widow girl 400 years ago being hit by a truck. He saved the girl’s life using his powers not knowing someone witnessed it. Came 2014, their paths crossed again. Cheon-Song-yi became an in-demand Hallyu star who went back to school for publicity stunt where she met his alien professor and neighbor Do Min-joon. Do Min-joon was unaware of Cheon Song-yi’s celebrity status, acted mean to her but always there when she gets in trouble. Cheon Song-yi’s was surprised by Do Min-joon’s odd treatment since she is used to having the VIP treatment because she is a celebrity. Little did she know, she was slowly drawn towards the mysterious professor and made her way in to his life. Cheon Song-yi’s childhood friend who has an unrequited love for her, Lee Hwi-kyung is also in the picture. Lee Hwi-kyung’s greedy older brother, Lee Jae-kyung framed up Cheon Song-yi for the killing of his model secret girlfriend and nemesis of Cheon Song-yi. In lieu to this, Cheon Song-yi’s career went downward spiral causing her to be depressed, bored but willing to clear up her name. Unbeknownst, Cheon Song-yi’s best friend, Yoo Se-mi is in love with Lee Hwi-kyung and their friendship was tested when Yoo Se-mi decided to go against Cheon Song-yi by getting all her projects. Do Min-joon ended up helping her which made them closer to each other and latter began falling in love with each other. Do Min-joon on the other hand has only 3 months left before he comes back to his home planet. Through the span of 3 months, Cheon Song-yi and Do Min-jook developed a mutual feeling but Do Min-joon was not vocal yet he expressed his love through actions. Finally, Do Min-joon admitted his feelings and his true identity to Cheon Song-yi. They decided to make the most out of his stay on earth until his departure comes. The fact is, Do Min-joon’s body is slowly deteriorating and he can’t use his powers properly and may result to his death thus he cannot stay longer on earth even if he wants to. The time has come, Do Min-joon disappeared. Cheon Song-yi was totally devastated and thought she’s getting crazy. She randomly see or hear Do Min-joon making her more miserable. It turns out that those were actual sightings of Do Min-joon who was on a warm hole recharging himself to make a comeback on earth. Three years has passed, Do Min-joon came back and they live happily ever after. (PS, Do Min-joon occasionally disappears to recharge and comes back thereafter)
- Jun Ji-hyun as Cheon Song-yi
A top Hallyu star who is very blunt and believes that she is every man’s type, until she meets her stoic neighbor, Do Min-joon. She soon develops feelings for him and tries to make him fall for her.
- Kim Soo-hyun as Do Min-joon
A very cold and distant being, he loved a girl who lived during the Joseon Dynasty for liking him as his true self, an alien. After the girl dies from saving him, he waits to return home. Living a lonely and isolated life for centuries, he one day meets Cheon Song-yi, a girl who, at her young age, looked exactly like the one he cared for 400 years ago and whom he once saved.
- Park Hae-jin as Lee Hwi-kyung
Song-yi’s childhood friend who has an unrequited love for her. He believes one day she will love him, but decides to back down for a while, supporting her through thick and thin as a true friend.
- Yoo In-na as Yoo Se-mi
Song-yi’s childhood best friend. Underneath her sweet smile, she is actually very envious of Song-yi and has had a crush on Hwi-kyung for a long time. She later becomes famous after Song-yi’s fall from the entertainment industry.
- Shin Sung-rok as Lee Jae-kyung
Hwi-kyung’s older brother, the potential heir of S&C Group. The main antagonist of the series, willing to do anything to silence Song-yi for a secret she knows about him.
- Ahn Jae-hyun as Cheon Yoon-jae
Song-yi’s younger brother, who first disapproves of her relationship with the neighbour but grows to likes Min-joon because of their shared interest in astronomy.
- Kim Chang-wan as Jang Young-mok
Min-joon’s lawyer. When he was young, He was saved by Min-joon from a suicide attempt and found out about his identity. He has been a loyal friend of him ever since.
- Na Young-hee as Yang Mi-yeon: Song-yi’s mother
- Oh Sang-jin as Yoo Seok: Se-mi’s older brother
A young and enthusiastic prosecutor looking into the Han Yoo-ra case. He gets to know about Do Min-joon’s real identity.
- Kim Hee-won as Park Byung-hee
A detective, who works together with Yoo Seok on the case of Han Yoo-ra. Knows about Do Min-joon’s identity.
- Jo Hee-bong as President Ahn: CEO of Song-yi’s talent agency
- Kim Kang-hyun as Yoon Beom: Song-yi’s manager
- Hong Jin-kyung as Bokja: comic book store owner/Song-yi’s school friend
- Kim Bo-mi as Min-ah: Song-yi’s stylist
- Lee Jung-gil as Lee Beom-joong: Jae-kyung’s father, the chairman of S&C Group
- Lee Il-hwa as Han Sun-young: Se-mi’s mother
- Sung Byung-sook as Hong Eun-ah: Hwi-kyung’s mother
- Um Hyo-sup as Cheon Min-goo: Song-yi’s father
- Lee Yi-kyung as Lee Shin: Lee Jae-kyung’s secretary
- Jo Se-ho as Chul-soo: comic book store customer/unemployed neighbor 1
- Nam Chang-hee as Hyuk: comic book store customer/unemployed neighbor 2
- Jo Seung-hyun as young Lee Hwi-kyung
- Kim Hyun-soo as young Cheon Song-yi
- Kim Hye-won as young Yoo Se-mi
- Jeon Jin-seo as young Cheon Yoon-jae