11:40 AM - During 4th period when all the students are in gym class, top student Jeong-hun (Yu Seung-Ho) walks into a classroom to find fellow classmate Tae-gyu (Jo Sang-Keun) brutally murdered. Jeong-hun and Tae-gyu has had a history of disliking one another. 11:50 AM - Da-jeong (So-ra Kang) 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 (Kim Dong-Beom) then takes part in finding the killer. When the 40 minutes are almost up, they then discover a brutal new fact to the case ...