Plot:

In the year 2007, before the main events of the drama, 11-year-old fraternal twin brothers Kim Woo-jin (Jung Ji-hoon) and Kim Bum-gyun (Kim Ye-joon), along with their neuroscientist father Dr. Kim Kyu-chul (Kim Joong-ki), witness the arrival of a female humanoid alien (Gong Seung-yeon). Out of pity and curiosity, the family brings the alien with them and adopts her as a member of the family. The twins become fond of her, especially Woo-jin who names her Byul.[a] By chance, Dr. Kim discovers Byul's secret: she had brought with her an advanced form of technology that can record, and even lock, memories and convert them into video. In order to closely study this alien technology, Dr. Kim isolates himself from his family to work secretly on Byul in what he calls the "Beta Project." Dr. Kim never goes back home, making the twin Bum-gyun think Byul has taken their father captive, though Woo-jin believes their father has abandoned them.

 

In 2017, Woo-jin (Yeo Jin-goo), now a 21-year-old college student in neuroscience, feels that a series of suicides in his university is somehow linked to Bum-gyun (Ahn Woo-yeon), who has been searching for aliens, particularly Byul. While in pursuit of the case, Woo-jin meets Han Jung-yeon (Gong Seung-yeon), a computer science student who, to his great shock, looks very much like Byul. She is also investigating the multiple suicides, all of which she thinks are actually murder.

 

In 2037, South Korea is now divided into Normal Earth,[b] a heavily polluted place where crimes are rampant, and Smart Earth,[c] a clean, peaceful, and crime-free city. Kim Joon-hyuk (Kim Kang-woo) is a Normal Earth crimes detective who tries to get into Smart Earth to investigate a case of twin brothers who went missing in 2017. In doing so, he starts to uncover the dark truth lurking behind Smart Earth being "crime-free."

 

Each episode of the drama, except the last, contains two parts: Part 1: Beta Project,[d] which contains a plot set in 2017, and Part 2: A Great New World,[e] which contains a plot set in 2037. The two plots merge in the twelfth and final episode of the series, titled Circle: One World.