Su Yan, of the Criminal Investigation Detachment of the Municipal Public Security Bureau, was once the most talented young detective in the police force. However, due to a "perfect closed-loop" suicide case three years ago - the death of his mentor Lu Chengyu, his career fell into a low point. He was transferred from the core case handling team and became a file administrator. In the past three years, she has never given up on investigating the suspicion of her mentor's death, firmly believing that the "Clue No. 7" That was deliberately erased from the scene is the key to uncovering the truth. Lu Chengyu left behind six clearly marked case files before his death, but the seventh one was missing. All traces related to "seven" were artificially destroyed. Until a bizarre secret room murder occurred, and the deceased was the last person Lu Chengyu came into contact with before his death. An old copper key with the number "7" engraved on it, exactly the same as three years ago, appeared at the scene, and the dust-laden mystery was completely broken. Su Yan was forced to return to the front line of handling cases, and partnered with the cold-blooded and resolute criminal investigation team leader Lu Shiyan. The two went from mutual suspicion to mutual trust. Following the copper key step by step, they discovered that this murder case was closely connected with the suicide case three years ago and an unsolved case many years ago. The "No. 7 Clue" is not only related to the cause of Lu Chengyu's death, but also hides a dark conspiracy that spans ten years and involves multiple interests. As the investigation progresses, people around him are in danger one after another, and clues are broken again and again. Su Yan gradually realizes that the death of his mentor is no accident. He has been involved in a huge trap, and the copper key engraved with "7" is only the first door to open the trap. The real danger has just begun.
