In Jiangcheng in 1938, Song Changyun, a clerk in the archives section of the pseudo police station, sorted out the case files of Japanese soldiers and traitors every day. No one knows that this taciturn young man is the sharpest code breaker in the underground party's single-line intelligence network. An intercepted secret telegram from Sato's special high school class made Song Changyun discover Jiangcheng's upcoming purge plan. Hou Wanjun, who went online, urgently activated the dead mailbox, but the password for the connection was discovered by Lu Yongnian - Hou Wanjun only had time to say half a sentence before he was arrested: "The password book is...". Song Changyun, who lost his online status, fell into isolation. He must use his gifted mathematical intuition to find the code book before the enemy within seventy-two hours, while avoiding the pursuit of Super High Section Chief Sato Masaki. Every decryption points to a traitor or a Japanese stronghold, but every action may never wait for the retreat signal after a single line breaks. When Jiang Caiheng appeared with new instructions, Jin Yaozu's guerrillas were waiting for him to send the coordinates of the firepower point, but Liu Qianying, his childhood friend, had already smelled something strange about him - Song Changyun only had the password in his hand and the bottom line of never involving the masses. Can this solitary knife cut a bloody path behind enemy lines?
