"Sirius" revolves around Identical twin brothers Eun Chang and Shin Woo. 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 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.