During Dae-young's life as the 18-year-old Go Woo-young, he learns that his daughter Shi-ah is working part-time in a convenience store without his permission and his son Shi-woo is being bullied at school by the basketball captain, Goo Ja-sung (Hwang In-youp).

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 doing his best to be more present to the family despite his appearance as an 18-year-old boy.

The series adaptation is directed by Ha Byung-hoon, who also helmed the similarly themed romantic fantasy drama Go Back in 2017.

[citation needed] 18 Again was originally scheduled to premiere on September 7, 2020, but it was postponed by two weeks due to the COVID-19 pandemic.