After a tragic accident leaves Momo brain-dead, neurosurgeon Ju Shin performs a forbidden surgery to save her.

Six months before the accident, Yong Joong learns who Ba Ra is as Ju Shin pushes Momo for marriage.

Mo-mo goes missing after a scuba diving accident. Ba-ra, a journalist, senses something suspicious about the case, while Yong-jung also feels an unsettling vibe from Ju-shin. Meanwhile, Ba-ra reveals the engagement of Ju-shin and Mo-mo in an exclusive interview without informing him, as Mo-mo calls 911 to report her mother's death.

Ran-hee's lifeless body is taken away, and Ju-shin mourns for Mo-mo. The funeral proceeds, but those around her cannot believe she would take her own life with no apparent reason. Meanwhile, Mo-mo, who now carries Ran-hee's soul, pretends to mourn while contemplating the possibilities of her new life.

Ju-shin begins to feel a growing sense of unease toward Ran-hee, who now has Mo-mo's appearance. Over a meal with Jin-ju, Ju-shin tries to gauge her feelings. Meanwhile, Yong-jung confesses his feelings to Ba-ra, and Ba-ra begins to see hallucinations of Mo-mo.

Ran-hee grows increasingly irritable as she tries to get closer to Ju-shin. However, Ju-shin seeks out Jin-ju, shows interest in her, and urges her to undergo the surgery. After meeting Gwang-cheol, her supposed biological father, Jin-ju vanishes without a trace, leaving behind only a single letter that shocks everyone.

Ju-shin successfully transplants Jin-ju into Mo-mo's body. As she begins life as Mo-mo, her biological father searches for her, waiting and refusing to believe she's dead. Meanwhile, Jin-ju offers Ba-ra a job to work by her side, and Ju-shin grows drawn to Ba-ra as he senses something is off with Jin-ju.

As Ju-shin looks deeper into Ba-ra, he finds himself increasingly drawn to her and even makes her an unthinkable offer. Ba-ra accepts Mo-mo's proposal, quitting her job to stay by her side, while Jin-ju's father continues his relentless search for her. Meanwhile, Jin-ju, as Mo-mo, discovers she's pregnant and hesitantly confides in Ju-shin.

Jin-ju seeks revenge on Ba-ra and tries to seduce Yong-jung while in Mo-mo's body. Meanwhile, Gwang-cheol continues to appear before Jin-ju, who now inhabits Mo-mo's body, pressuring her at every turn. Amid it all, Ju-shin and Ba-ra find themselves drawn into a tense, ambiguous moment as they head into the bedroom together.

As Jin-ju reels from Gwang-cheol's shocking visit, she cuts ties with Ju-shin and turns to Yong-jung, with whom she shares a past. She urges him for his full support, and they go to a karaoke room for drinks. Meanwhile, certain of his feelings, Ju-shin shocks Ba-ra with a marriage proposal.

Yong-jung warns Ba-ra to keep her distance from Ju-shin, but the two only grow closer. As Ju-shin continues to pursue Ba-ra, Jin-ju learns that Ju-shin had proposed to Ba-ra and gets overcome with fury. Suddenly, panic overcomes Ju-shin as he learns that Ba-ra is secretly planning to go away.

Yong-jung tells Ba-ra they should set a wedding date, but Ba-ra leaves him and heads to Ju-shin's house. Meanwhile, Gwang-cheol begins volunteering at Nua General Hospital, and Jin-ju is delighted when she hears something unexpected from Yong-jung.

Ba-ra is devastated to learn that Jin-ju is actually Mo-mo and confronts her with a final plea. Despite this, Jin-ju and Yong-jung go through with their wedding. Meanwhile, Ju-shin asks Ba-ra if she would consider undergoing a brain change into Mo-mo's body to be with Yong-jung.

Yong-jung and Ju-shin discover Ba-ra's pregnancy. After seeing Mo-mo wearing Jin-ju's earrings, Gwang-cheol's suspicions deepen. At Paul Kim's house, Ba-ra stumbles on a picture that looks just like her. Meanwhile, Yong-jung and Mo-mo's relationship hurtles toward an irreversible breakdown.

Jin Ju tries to persuade Ba Ra to undergo a brain change as hidden truths surface and confusion spreads.

Consumed by grief, Gwang Cheol confronts Ju Shin as long-hidden secrets surrounding Jin Ju begin to unfold.