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 help Da-won escape from the 3.5th period, Ah-jin tries to solve the mystery hidden there.