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.

Ri-jin encounters virtually all of Do-hyun's personalities in one night. Do-hyun makes a shocking revelation when he finally finds out who Writer Omega truly is.

Se-ki grows angry when he discovers that Ri-jin has been spending time with Do-hyun. Ri-jin grows worried when she's told that Do-hyun may never return to recover his forgotten memories.

Ri-jin encourages Se-ki to share the painful childhood memory with Do-hyun, and discovers what Ri-ohn has been hiding in his room.

Do-hyun vows to find the other mysterious child that was also at the Seungjin mansion when he was a boy. Ri-ohn is caught off-guard when the teenage girl personality Yo-na suddenly appears.

Ri-jin discovers that her own childhood memories are emerging in the pursuit to recover Do-hyun's memories. Do-hyun finds out the truth of what happened in that basement during his childhood days.

Rattled by the truth of the memories he shares with Ri-jin, Do-hyun is committed to living the rest of his life in repentance. Ri-ohn asks Do-hyun to step in so that Ri-jin doesn't recall her childhood memories.

Do-hyun and Ri-jin embark on a trip together before their contract is terminated. As Ri-jin starts to recall more of her past, the memories bring with it deeply-buried pains, and most importantly, the truth.

More memories of the past comes to light for Do-hyun and Ri-jin. Do-hyun confronts his grandmother about the fire that took place twenty-one years ago and learns who truly started the fire.

Do-hyun's personalities appear over the course of one night when he learns the entire truth. However, a past threat appears to break his resolve once more.

Ri-jin and Do-hyun embark on their cathartic journey to deal with their shared painful memories. But just as they begin, another personality shows up to threaten their progress.

Do-hyun bids farewell to all of his personalities with the help of a new personality, and finally confronts his family about the past.