There were three murder cases in Z town. On the evening when the murderer killed the third person, the police found the fourth body. After police investigation, it was found that this was the murderer of the first three cases - Shi Lei, a freshman at xx High School (Youle's childhood friend). After You Le learned about Shi Lei's motive for killing, he didn't believe that his childhood friend would do such a thing, so he and his brother (You Tang) began to investigate the strangeness.