Taken from wiki:
The human body is capable of all sorts of things to survive difficult situations. Cha Do Hyun (Ji Sung) is a third-generation business heir who developed dissociative identity disorder (previously known as multiple personality disorder) in the aftermath of several life-threatening traumatic events. He tries to regain control over his life with the help of Oh Ri Jin (Hwang Jung-eum), a first-year psychiatric resident who helps him secretly. But Ri Jin’s twin brother, Oh Ri On (Park
Seo-joon), is a writer who is determined to uncover the unscrupulous lives of the rich and starts following Do Hyun around. Can Do Hyun take control over his condition before one of his seven personalities takes control over him instead?