A family of three moves into the Oryun Mansion Complex, and Ju-won meets San-ha, the brother she always wanted, for the first time. Despite Ju-won's eager wish to get to know him, San-ha has trouble accepting her due to his family's situation. Meanwhile, San-ha's father meets Kang Seo-hyun, a single mother with a son named Hae-joon.

After growing up together as a family, Ju-won, San-ha, and Hae-joon go to high school together. San-ha and Hae-joon are hot with the girls, causing Ju-won to deal with the onslaught of requests from the girls to deliver their love message to them. Meanwhile, Ju-won longs to live under one family name through adoption, which the others think is absurd. Later, Hae-joon catches a suspicious man taking photos of him.

A man visits Hae-joon at school and reveals that he is Hae-joon's birth father. Although Hae-joon is initially speechless, he feels relieved to learn that his father has not abandoned him. To find out more about his father, he and San-ha talk with Aunt Yi-hyun to gain insight into his father's character before meeting him in person and uncovering the reasons for his sudden return after all these years.

Ju-won feels exhilarated about going on a date with a boy who is good-looking and smart, but she soon gets disappointed by what he says to her. San-ha and his father are upset by an unwelcome visitor who reminds San-ha of memories he doesn’t want to recall. A surprise present from Hae-joon’s biological father brings about a conflict between Hae-joon and his colleague.

With Hae-joon’s birthday around the corner, San-ha and Hae-joon get into a big fight. To help them reconcile, Ju-won plans to take them to a rail bike. However, many misunderstandings develop as Ju-won collapses from a cold, and rumours between Dal and Hae-joon spread at the school. Meanwhile, everyone gets ready as Yi-hyun plans to join the birthday party.

Hae-joon’s mother, Seo-hyun, unexpectedly visits Yoon’s Kalguksu to repay her debt, leaving Jung Jae puzzled as to what to do with it. Meanwhile, So-hee goes missing after leaving home to meet San-ha. Her disappearance reignites the unresolved conflict between Moo-sung and Jeong-hee, resulting in San-ha's decision to distance himself from the family.

Hae-joon finds out that his mother had come to pay back her debt, but did not stay long enough to see him. After CSAT, misunderstandings between San-ha, Ju-won, Hae-joon, and Dal build up as they are clueless about each other’s true feelings. Meanwhile, Hae-joon’s birth father watches his son’s basketball game and suggests he study further in the US.

San-ha stays in Seoul to look after his step-sister while his mother recovers from surgery. Ju-won goes to Seoul alone to surprise him without telling the rest of the family. Meanwhile back at home, Jeong-jae collapses from overworking, scaring everyone, especially Hae-joon, who makes up his mind about going to the US.

Ju-won is shocked to find out both San-ha and Hae-joon have made up their mind to leave for Seoul and the US. Feeling a part of the family will be missing, she can’t help but cry. Jeong -jae also feels bad sending the boys away and feels empty in heart. Ten years passed, San-ha and Hae-joon came back as two grown-ups.

San-ha and Hae-joon come back on Ju-won’s birthday after ten years. Though they haven’t seen each other for years, unlike what Ju-won had imagined, she feels indifferent about their return and is not happy to see them. Meanwhile, the family finds out a shocking truth about Hae-joon at Ju-won’s birthday celebration.

San-ha’s sudden confession startles Ju-won. While she tries to deny any romance between them and stay as siblings, she is overcome with mixed feelings. Hae-joon jokingly suggests getting married to Ju-won, upsetting San-ha. Meanwhile, a surprise visit from Seo-hyun shocks Jeong-jae as an unexpected turn of events takes place.

San-ha and Ju-won finally confirm their feelings for one another. However, they soon become worried about how to break the news to their family and friends. Meanwhile, Hae-joon tries to pay back Jeong-jae for everything he has done for him, but some misunderstandings develop as Hae-joon’s source of money is revealed.

Hae-joon finds out that San-ha is struggling with insomnia and taking medication. So-hui comes to visit San-ha alone without telling Jeong-hee and asks him to stay with her and Jeong-hee. Dal keeps trying to hide her feelings for Hae-joon. Meanwhile, a big secret between Ju-won and San-ha is about to be revealed by none other than Hae-joon.

Hae-joon is filled with a deep sense of betrayal as he watches San-ha and Ju-won on the verge of kissing, struggling to understand how Ju-won could accept San-ha as a boyfriend. He’s haunted by the fear that, like everyone else before them, they too will eventually leave him. Meanwhile, though he knows where his mother is, Hae-joon watches her from a distance, and each glimpse only fuels his growing hatred for her.