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...