11:40 AM - During 4th period when all the students are in gym class, top student Jeong Hun walks into a classroom to find fellow classmate Tae Gyu brutally murdered. Jeong Hun and Tae Gyu have had a history of disliking one another. 11:50 AM - Da Jeong walks into the same classroom and discovers Jeong Hun with blood covering his clothes standing near the murdered body. Da Jeong is a mystery buff herself and loves to use her deductive reasoning. She realizes that Jeong Hun is the prime suspect, but doesn't believe that he killed Tae Gyu. There's only 40 minutes before next period, in which all the students will come into the classroom and also discover the dead body. 12:05 PM - Jeong Hun and Da Jeong frantically search for the real killer before the next period starts. With Da Jeong using her deductive reasonings they get closer to finding the identity of the true killer. Jeong Hun's best friend Do Il then takes part in finding the killer. When the 40 minutes are almost up, they then discover a brutal new fact to the case ...