Cha Dohyeon 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 Rijin is a first-year psychiatric student who runs into one of Dohyeon's many personalities while looking for one of her patients.

Falling in love at first sight with Rijin, Shin Segi vows to get rid of his host Dohyeon. Dohyeon looks for ways to make sure to keep his various personalities in check while he assimilates to life in Korea again.

Dohyeon feels guilty when he finds out that he inadvertently put Rijin in danger because one of his alter egos. Rijin figures out the truth of his condition and sympathizes with him.

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

Unable to tell Dohyeon and his other personalities apart, Rijin gets confused about who she's beginning to have feelings for. Dohyeon is warned that Segi's personality is growing stronger.

After getting through a life-threatening situation with Ahn Yoseop, Rijin is left with the decision to either leave for America or become Dohyeon's doctor. Cha Gijoon tries to look into why his cousin sees a psychiatrist.

Rijin discovers a new personality in Dohyeon when he learns about one of Omega's stories. Han Chaeyeon begins to feel jealous that Dohyeon might like Rijin.

Rijin encounters virtually all of Dohyeon's personalities in one night. Dohyeon makes a shocking revelation when he finally finds out who Writer Omega truly is.

Segi grows angry when he discovers that Rijin has been spending time with Dohyeon. Rijin grows worried when she's told that Dohyeon may never return to recover his forgotten memories.

Rijin encourages Segi to share the painful childhood memory with Dohyeon, and discovers what Oh Rion has been hiding in his room.

Dohyeon finally recovers a crucial piece of his forgotten memories and is now determined to become stronger and bear the burden. Chaeyeon is conflicted about her upcoming engagement to Gijoon.

Dohyeon vows to find the other mysterious child that was also at the Seungjin mansion when he was a boy. Rion is caught off-guard when the teenage girl personality, Ahn Yona, suddenly appears.

Rijin discovers that her own childhood memories are emerging in the pursuit to recover Dohyeon's memories. Dohyeon finds out the truth of what happened in that basement during his childhood days.

Rattled by the truth of the memories he shares with Rijin, Dohyeon is committed to living the rest of his life in repentance. Rion asks Dohyeon to step in so that Rijin doesn't recall her childhood memories.

Dohyeon and Rijin embark on a trip together before their contract is terminated. As Rijin 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 Dohyeon and Rijin. Dohyeon confronts his grandmother about the fire that took place twenty-one years ago and learns who truly started the fire.

Dohyeon'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.

Rijin and Dohyeon 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.

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