In the late 1970s, on the steam train from Ningyang to Harbin, a young police officer Wang Xin is on duty and shuttling among crowds of passengers. He mistakes an old railway policeman Ma Kui as a fugitive, and their conflict gradually develops into a mentor-and-apprentice friendship. From disliking each other to sympathizing with each other, Wang Xin and Ma Kui fight side by side on the front line of railway public security. They crack down on theft, kidnappings, drug trafficking, and fraud, always prioritizing the safety of the passengers and selflessly safeguarding the trains traveling from the south to the north. From steam locomotives to electric locomotives to high-speed trains, the new and old-generation railway police officers Wang Xin and Ma Kui witness the development of China's railway system and China's rapid changes as they uphold the spirit of the police.