With Hae-jun's birthday around the corner, San-ha and Hae-jun 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-jun spread at the school. Meanwhile, everyone gets ready as Yi-hyun plans to join the birthday party.

Hae-jun'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 Jung-hee, resulting in San-ha's decision to distance himself from the family.

Hae-jun 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-jun, and Dal build up as they are clueless about each other's true feelings. Meanwhile, Hae-jun's birth father watches his son's basketball game and suggests he study further in the US.