A Student Police Officer Solves Cases and Tracks Japanese Spies in Republican-Era China

Zhang Yuchen, a student at the Public Security University, finds an old brush while treasure hunting in Panjiayuan. To his surprise, the brush can draw a door on a wall that lets him transmigrate to the Republican era. He starts out solving cases and eventually tracks down Japanese spies. As events continue to escalate, a whole series of incidents is set in motion. Zhang Yuchen realizes he needs help, but should he come clean to the authorities and seek cooperation with the government, or recruit his own people...?