Wu Tao, the network administrator at Research Institute No. 3, was poisoned and killed, drawing the attention of the National Security Bureau. After Han Shan and the others investigated, they determined it was a spy case. A USB drive containing top-secret technical information had fallen into the hands of Li Qicai, a mechanic at a machinery plant, who planned to trade it to a spy known as “Yellow Guest.” While Han Shan was tailing Li Qicai, Li Qicai was suddenly killed in a car accident. Han Shan was also ambushed and injured, and ended up in the hospital. At this point, his superiors assigned Yang Xiaodu, a female intelligence officer, to help investigate the case. Posing as a reporter, Yang Xiaodu visited Han Shan in the hospital and mentioned a spy named “Time Killer,” arousing Han Shan's suspicions. Yang Xiaodu went alone to Liu Zhen's home to continue investigating, where she learned that Li Qicai and Wang Quanyou were twin brothers who had been separated at birth because their family was poor. Before Wang Quanyou disappeared, he wrote the simple sequence “1234567” on the last page of his diary. Yang Xiaodu persuaded Liu Zhen to join her in searching for the secret behind the numbers. Li Qicai and “Time Killer” made a deal at the head of a bridge...
