His rival, Tae-gyu (Jo Sang-geun), takes any and every opportunity to knock him down both verbally and physically, and the two are known enemies amongst the other students.

Jung-hoon walks away from the situation, but shortly after the beginning of 4th period class he returns to find Tae-gyu dead.

The boy has been stabbed repeatedly, and still in shock, Jung-hoon picks up the bloody knife off a nearby desk just as Da-jung (Kang So-ra) shows up, catching him in a most incriminating position.

Fortunately for him, she not only believes his innocence, but offers to help him to solve the mystery and catch the real killer – a task made particularly urgent by the fact that in 40 minutes the rest of the class will return and the body will be discovered.

Originally, actress Kim So-eun was going to play the role of Dajung but she was replaced with Kang sora because of Kim's filming schedules for the TV series He Who Can't Marry.