Cha Do Hyun is in line to be the successor of Seung Jin 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. Seung Jin 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 finally recovers a crucial piece of his forgotten memories, and is now determined to become stronger and bear the burden. Chae Yeon is conflicted about her upcoming engagement to Ki Joon.

Do Hyun vows to find the other mysterious child that was also at the Seung Jin 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.