A family moves into an apartment complex; the daughter, Ju Won, meets San Ha, her new stepbrother, who struggles to accept her due to his family's circumstances.

After growing up together, Ju Won, San Ha, and Hae Jun attend high school; San Ha and Hae Jun attract female attention, leading to Ju Won relaying love messages; Ju Won desires adoption under one family name.

A man claims to be Hae Jun's birth father, visiting his school. Initially speechless, Hae Jun feels relief at learning his father didn't abandon him.

Ju Won's date disappoints her; San Ha faces an unwanted visitor, dredging up unwanted memories; Hae Jun's biological father's gift causes conflict between Hae Jun and colleague.

Hae Jun's birthday is near, but he and San Ha fight; to fix things, Ju Won plans a rail bike trip, but he gets sick; misunderstandings arise at school about Dal and Hae Jun, while Yi Hyun prepares for the birthday party.

Three young adults who were raised together as kids reunite, but they must work through feelings of abandonment and budding romantic love.