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.