Cha Do-hyeon 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-hyeon's many personalities while looking for one of her patients.

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

Do-hyeon 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-hyeon declines treatment and withdraws from people and work. Seungjin Group harbors a deep family secret kept hidden from Do-hyeon, who also discovers a new personality has emerged.

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

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-hyeon's doctor. Cha Gi-joon tries to look into why his cousin sees a psychiatrist.

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

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

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

Ri-jin encourages Se-gi to share the painful childhood memory with Do-hyeon, and discovers what Oh Ri-on has been hiding in his room.

Do-hyeon 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 Gi-joon.

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

Ri-jin discovers that her own childhood memories are emerging in the pursuit to recover Do-hyeon's memories. Do-hyeon 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-hyeon is committed to living the rest of his life in repentance. Ri-on asks Do-hyeon to step in so that Ri-jin doesn't recall her childhood memories.

Do-hyeon 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 come to light for Do-hyeon and Ri-jin. Do-hyeon confronts his grandmother about the fire that took place twenty-one years ago and learns who truly started the fire.

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