Every class in school has its own class motto. Cheol-gyu, a school physics teacher, asks his students with physics-associated equations but shoots immediately if a student cannot answer. The last student remaining was No.7 student, who keeps on giving correct answers to Cheolgyu’s questions. Then, the No.7 asks back to him but he couldn’t find the right answer. Spotting Cheol-gyu was hesitating, the No.7 immediately finds his own gun and points it towards him. What did the No.7 ask to Cheol-gyu? Could he finally give the answer to it? There unexpected person comes in to the class through its backdoor.....