Plot

High school guys must find the killer of their classmate before the next class start, in 40 minutes.

4th period murder mystery. During fourth period, high schooler Da-Jeong finds her classmate Tae-Gyu dead in a classroom and Jeong-Hun standing in front of him with blood on his clothes. In order to stop Jeong-Hun from becoming the prime suspect, Da-Jeong must help him find the real murderer, who is still inside the school building, under 40 minutes before period four ends.

When a student is stabbed at school, Jung Hoon (Yoo Seung Ho) is discovered at the scene of the crime by Da Jung (Kang So Ra). As a result these two form a team to uncover the murderer, but they must do this before the end of fourth period.

Jung Hoon is a diligent and hard working student. Meanwhile Da Jung, often referred to as curtain girl, is often immersed in Crime novels. Two weeks prior during class, a student starts frothing from the mouth and collapses. As the class rush forward to aid him Da Jung is seen taking photos of the crime. It is later shown through gossiping that the student passed away.

Tae Gyu is Jung Hoons rival, when he takes a photo of a girl without the girls permission, a fight ensues between him and Jung Hoon leaving them to serve detention. When the rest of the class goes to PE, the two are left alone. Jung Hoon leaves to go to the bathroom but as he returns he finds the body of Tae Gyu who has been repeatedly stabbed in the stomach. It is here that Da Jung finds him. As a result the two band together to discover the murderer before the end of Fourth period, before everyone returns to find the body. But soon they may discover that the muderer is willing to kill again to prevent being discovered.