Wishing he could go back in time and fix his life, Dae-young's 37-year-old body miraculously morphs into the body he had when he was 18 years old (Lee Do-hyun) but retains his 37-year-old mentality. He starts living a new life under his alias "Go Woo-Young" and attends Serim High School, which is his twins' school, and also the school he and Da-jung attended 18 years ago. During Dae-young's life as the 18-year-old Go Woo Young, he learns that his daughter Si-a is working part-time in a convenience store without his permission and his son Si-u is being abused at school by basketball captain Goo Ja-sung (Hwang In-youp). Dae-young also learns about his son's interest and talent in basketball that Si-u always hides from him. Dae-young eventually realizes the true reason behind his family's crumbling bond and gets to know more about his twins. He resolves to use this chance to protect his kids by befriending them and to do his best to be more present to the family despite his appearance as an 18-year-old boy.