The story takes place in Yunlan City on the southwest border of China and Jianghai City in the inland city from 2018 to 2023. Yunlan City, as the forefront of anti-drug control, has been fighting against the "Golden Crescent" drug infiltration all year round. The domestic "cage" drug trafficking network is entrenched in the mountains and focuses on traditional heroin smuggling. Jianghai City is facing the spread of new synthetic drugs. Transnational drug trafficking groups have established underground laboratories through smuggling of chemical raw materials, forming a "production-distribution-consumption" black industry chain. The police from the two places established a joint task force, codenamed "Undercurrent", to try to cut off the drug flow at the source.
