11:40 AM - During 4th period when all the students are in gym class, top student Jung-Hoon (Yoo Seung-Ho) walks into a classroom to find fellow classmate Tae-Gyu (Jo Sang-Keun) brutally murdered. Jung-Hoon and Tae-Gyu has had a history of disliking one another. 11:50 AM - Da-Jung (Kang So-Ra) walks into the same classroom and discovers Jung-Hoon with blood covering his clothes standing near the murdered body. Da-Jung is a mystery buff herself and loves to use her deductive reasoning. She realizes that Jung-Hoon 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 - Jung-Hoon and Da-Jung frantically search for the real killer before the next period starts. With Da-Jung using her deductive reasonings they get closer to finding the identity of the true killer. Jung-Hoon's best friend Do-Il (Kim Dong-Beom) then takes part in finding the killer. When the 40 minutes are almost up, they discover a shocking new fact to the case...