"Sirius" revolves around Identical twin brothers Eun-Chang and Shin-Woo (both played by Seo Jun-Young). When they were teens, Eun-Chang was a bright and popular kid, but went to jail because of Shin-Woo. Since then, his life has changed completely. Meanwhile, Shin-Woo always felt inferior to his brother and his feelings for his brother became more complicated after his brother was imprisoned.

Shin-Woo is now the chief detective of a police investigation team. He works mainly with  drug investigations. Eun-Chang now works as an errand man at a hostess bar. A conspiracy is implemented by Ko Seok-Min (Ryu Seung-Soo) and the twin brothers change positions. Eun-Chang now acts as Shin-Woo in the police department. Shin-Woo is shocked that his brother stole his identity and begins to take actions to get his identity back. Meanwhile, Ko Seok-Min proposes a deal with Eun-Chang.