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.
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.
Hae Jun's mother, Seo Hyun, visits Yoon's Kalguksu to repay a debt, confusing Jung Jae. Meanwhile, So Hee goes missing after meeting San Ha, causing tension between Moo Sung and Jung Hee, leading San Ha to distance himself from the family.
After his mother leaves again, Hae Jun and friends face misunderstandings about unspoken feelings. His father wants him to study in the US after seeing him play basketball.
Three young adults form an unbreakable bond, finding solace in their makeshift family. Despite facing trauma, they create cherished memories under the guidance of their fathers.
On Ju Won's birthday, San Ha and Hae Jun return after a decade, leaving Ju Won indifferent despite expectations. A shocking truth about Hae Jun surfaces during the celebration.
San Ha confesses romantic feelings for Ju Won, leaving them conflicted. Hae Jun jokingly proposes to Ju Won, upsetting San Ha. Seo Hyun's surprise visit shocks Jung Jae, leading to unexpected events.
San Ha and Ju Won confess their love but worry about telling family and friends. Hae Jun tries repaying Jung Jae but misunderstandings arise over Hae Jun's money source.
Hae Jun discovers San Ha's insomnia and medication. Jung visits San Ha privately, asking him to stay with her and Jung Hee. Dal conceals her feelings for Hae Jun as a secret between Ju Won and San Ha is about to be unveiled by Hae Jun.
Hae Jun feels betrayed watching San Ha and Ju Won nearly kiss. He fears they'll abandon him like others before. Despite knowing his mother's whereabouts, he watches her from afar, fueling his hatred towards her.
San Ha gets injured, rushed to the ER. Jung Jae and Dae Wook visit. Hae Jun resumes basketball. Dal is devastated. An unwelcome visitor arrives at Ju Won's bakery.
San Ha seeks Jung Jae's approval to pursue Ju Won. Ju Won and Hae Jun play matchmakers for Jung Jae and Seo Hyun, while Dal goes on a blind date per Jae Eun's suggestion. Hae Jun intervenes, and Jung Hee plans to move to UK with So Hee.
