A lawyer is obsessed by a TV advertisement's two models, Yau Yuk-nam and Chu Kit-Man. He schemes an elaborate plot to sexually assault Chu and get away with it. With the help of a philanthropic triad boss, Yau uses her body to lure him into a trap.

The final entry in Michael and Johnny Mak's Long Arm of the Law series, Underground Express once again tells a story of gangsters versus police, focusing on the difference between how both factions act depending if they're coming from Hong Kong or Mainland China.