On the eve of the Battle of Songhu in 1937, Ling Xiao, a top modern agent, encountered an explosion while performing an anti-terrorism mission. Due to the activation of his ancestral bronze key, he traveled to Shanghai, where the situation was complicated, and was accidentally recruited into the military system. Using the boss of "Xinya Trading Company" as a cover, he secretly investigated the secret of the key.
He got acquainted with Su Manqing, who appeared to be a socialite but was actually an underground member of the Communist Party. From mutual suspicion to fighting side by side, the two developed feelings for each other. They also established cooperation with Chen Mo, the leader of the military action team who was upright but dissatisfied with the corruption of the Kuomintang. With the help of archaeologist Mr. Zhou, Ling Xiao learned that the bronze key was related to the "Eastern Defense Blueprint" that could change the situation of the war. Yamada Yuichi, the head of the Japanese Army's Special High School Course, was eyeing the key and the blueprint, and set up crises many times.
After going through many dangers, Ling Xiao gave up his obsession with returning home and joined the underground party, uniting various forces to find the fragments of the blueprint. Finally, the complete blueprint is found in the century-old house, and the ultimate showdown with Yamada and his "Shadow Guards" begins. The key is used to activate the ancient formation to reverse the situation and defeat Yamada. Ling Xiao chose to stay in the Republic of China and guard Shanghai with Su Manqing, and the glimmer of the bronze key also heralded the coming of new challenges.
