Jung-Joo (Ko So-Young) is 30-years-old, works in the fashion industry (but only as a low-level assistant) and just can’t seem to get things right in her life. One day she runs into high school friend Tae-Hoon (Lee Beom-Soo). Back in her high school days, Tae-Hoon had a huge crush on her, but Jung-Joo ignored him because she thought he was boring. Instead, Jung-Joo fell in love with Jo Hani, a rebellious student who eventually dumped Jung-Joo. Now, Tae-Hoon is the CEO of a successful technology company. As Jung-Joo lays in her bed and thinks back upon all the dumb things she did as a teen, a magical computer program appears on her laptop. The program allows Jung-Joo to travel back in time and Jung-Joo decides to revisit her high school days.