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Ru Bi is thrown into turmoil when Gyeong Min, the ex-lover she parted with five years ago, suddenly reappears as the new head of her division. Meanwhile, Ru Bi's younger sister Ru Na, a reporter at a cable broadcasting station, is dating In Su, a talented producer. In Su dreams of a simple, ordinary marriage, but Ru Na harbors far greater ambitions, setting the two on quietly diverging paths.
Gyeong Min asks Ru Bi for another chance, but she firmly pushes him away, set on not wavering again. Yet the moment she turns away, her heart grows tangled. Though she thought it was over, lingering feelings keep resurfacing, leaving her confused about what she truly wants. Meanwhile, In Su finds a curious mole on the sole of Ru Na's foot. But Ru Na's mind is heavy with another worry that she might be pregnant. Unsure how to face it or whom to confide in, she sinks into anxiety.
The pregnancy is confirmed, leaving In Su overjoyed while Ru Na sinks into despair. When Gyeong Min's confession drives Ru Bi to consider leaving the company, Gyeong Min calls her out and takes her to a place full of their shared memories. There, he confesses to her once more, stirring up the feelings Ru Bi has been trying so hard to bury.
Ru Na quietly visits the obstetrician on her own and ultimately decides to go through with an abortion. Meanwhile, Gyeong Min tells his family about his relationship with Ru Bi, and his father, Chairman Bae, heads to the company to see Ru Bi for himself, only to meet with an accident along the way.
When In Su suggests she quit her job and get married, Ru Na refuses to have the baby, prompting In Su to declare a breakup. Meanwhile, Chairman Bae meets Ru Bi and gives his blessing for the marriage.
Chairman Bae and Il Sun give their blessing for Gyeong Min and Ru Bi's marriage. Ru Na heads to the hospital for the abortion, but when the doctor advises against the procedure, she clings to In Su, desperately begging for his help.
Gyeong Min visits Gil Ja's place to ask for marriage approval. Watching from afar, Ru Na is overwhelmed with misery and mistreats Ru Bi. At Ru Bi's request, Ru Na goes to the company, where she accidentally meets Gyeong Min.
The day of Ru Bi and Gyeong Min's family meeting has arrived. Meanwhile, Cho Rim and Dong Pal chase the scammer who once stole her money in a taxi pursuit. And after the family meeting, Ru Na continues to lose her way.
On Ru Bi and Ru Na's birthday, the two sisters set off for Seoul together. Ru Na pushes Ru Bi to hand over her brand new outfit and forces her ruby ring off her finger. With Ru Na now dressed as Ru Bi and wearing her ring, the trip takes a frightening turn as Ru Na takes the wheel and sends them both into a dangerous drive.
After waking up from surgery, Ru Na is thrown into confusion when everyone around her begins calling her Ru Bi. As she tries to process what is happening, she learns that Ru Bi is in a coma. Seeing Gyeong Min sitting faithfully at Ru Bi's side, refusing to leave, something stirs inside Ru Na. A dark temptation begins to grow within her, one she finds difficult to push away.
In Su rushes to the hospital after hearing about the accident. He is shocked to learn that Ru Bi, not Ru Na, is pregnant. Meanwhile, Il Sun goes to Chuncheon to meet Gil Ja.
After her treatment, Ru Bi visits Gyeong Min's family, hoping to finally be accepted. But his grandmother still keeps her distance. As the engagement begins to shake, Il Sun urges Gil Ja to reconsider the marriage. Meanwhile, In Su notices the mole on Ru Na's foot and starts questioning the sisters' identities. Desperate to protect her new life, Ru Na begs him to keep her secret just once.
Gyeong Min visits Gil Ja's place to ask for marriage approval. Watching from afar, Ru Na is overwhelmed with misery and mistreats Ru Bi. At Ru Bi's request, Ru Na goes to the company, where she accidentally meets Gyeong Min.
It is the day of Ru Bi and Gyeong Min's family meeting. Meanwhile, Cho Rim and Dong Pal chase the scammer who once stole her money in a taxi pursuit. And Ru Na continues to lose her way.
Ru Na is deeply relieved to find out that Ru Bi has woken up with no memory of who she is. Seeing Ru Bi awake in Ru Na's face, In Su is consumed by the unbearable truth. Determined to stay in control, Ru Na works to convince Ru Bi that she is Ru Na, doing everything she can to make that identity stick.
Ru Na feels the weight of Ru Bi's memory loss, torn between guilt and the need to protect everything she has built. Her unease grows the next day when In Su shows up at Gyeong Min's home, threatening to disturb the fragile stability she has been holding onto. At the same time, Ru Bi begins psychological treatment.
On a trip to Chuncheon, Ru Bi runs into Yeon Ho and his friends. Mistaking her for Ru Na, Yeon Ho makes unwanted advances and Ru Bi narrowly avoids an uncomfortable situation. Ru Na is fully absorbed in her new life, enjoying every aspect of being a daughter-in-law in a wealthy family.
Ru Na is rattled when she is told to return to work, and her anxiety deepens after receiving a photo from In Su. Meanwhile, In Su is violently attacked by unknown assailants outside his home. At the office, Ru Na comes face to face with both In Su and Ru Bi at the same time.
Gyeong Suk gets deeply upset when Gyeong Min defends Ru Na. Gyeong Min himself is actually growing deeply disappointed in Ru Na because of her drinking and excessive shopping. Later on, Ru Bi and In Su arrive at the exact location where Gyeong Min and Ru Na are spending time together.
In Su and Se Ra are chased by a mysterious car. In Su gets angry when he finds out that Ru Na was behind it. Because of Ru Na's mistake, JM Group faces serious financial damage.
