Carol is a 17-year-old teenager, the daughter of a Brazilian dancer and a South Korean, who lives a challenging journey of overcoming, acceptance and self-knowledge after the death of her mother and the abandonment of her father, who decides to return to South Korea. In a conflicting relationship with her ancestry, she denies knowing any topic better than South Korean culture, but her life changes after a unique discovery: her wardrobe is actually a magic portal that connects her room to a group of k-pop of great success at the moment, ACT.
In the prestigious Shinha High School, there is a secret hidden between the third and fourth periods. There are 3.5 classes where time goes 10 times slower and 10 minutes becomes 100 minutes! Baek Ah-jin accidentally enters the 3.5th period and meets Kang Da-won, a male student who was trapped there. To escape Dawon from the 3.5th period, Ahjin solves the mystery hidden there. But there are people in the school who try to hide 3.5 classes...