Cha Do-hyun is in line to be the successor of Seungjin Group. No one in his family is aware that he's been suffering from dissociative identity disorder for years following a family tragedy. Oh Ri-jin is a first-year psychiatric student who runs into one of Do-hyun's many personalities while looking for one of her patients.

Falling in love at first sight with Ri-jin, Se-ki vows to get rid of his host Do-hyun. Do-hyun looks for ways to make sure to keep his various personalities in check while he assimilates to life in Korea again.

Do-hyun feels guilty when he finds out that he inadvertently put Ri-jin in danger because one of his alter egos. Ri-jin figures out the truth of his condition and sympathizes with him.

Considering his instability a risk to himself and others, Do-hyun declines treatment and withdraws from people and work. Seungjin Group harbors a deep family secret kept hidden from Do-hyun, who also discovers a new personality has emerged.

Unable to tell Do-hyun and his other personalities apart, Ri-jin gets confused about who she's beginning to have feelings for. Do-hyun is warned that Se-ki's personality is growing stronger.

Determined to keep Se-ki from taking over his life, Do-hyun asks Ri-jin to become his personal doctor. ID Enter is determined to track down Writer Omega's identity and works.

After getting through a life-threatening situation with Ahn Yo-seob, Ri-jin is left with the decision to either leave for America or become Do-hyun's doctor. Ki-joon tries to look into why his cousin sees a psychiatrist.

Ri-jin discovers a new personality in Do-hyun when he learns about one of Omega's stories. Chae-yeon begins to feel jealous that Do-hyun might like Ri-jin.