In the early years of the Republic of China, a series of mysterious events took place in the Nanyang region. The story follows Zhang Haiyan and Zhang Haixia, two secret agents from the Southern Archives, who investigate an enigmatic case involving a conspiracy of warlords using the poisonous plant "Dusk Grass". As they face life-threatening dangers, the clues begin to align, leading them to discover that all of them converge on a massive ship, the Nan'an.