Unmatchable Match (1990)

Psycho bad guy Shing Fui-On arranges a bloody diamond heist, then gets Triad Michael Chan involved in the deal to shift them, but the deal goes sour and Chan ends up on the run from the cops and the robbers and the rest. Stephen Chiau is an undercover cop who is assigned to ingratiate himself with Chan and track down the missing diamonds. Eventually winning his trust, Chiau ends up in the undercover cop's usual situation, where he finds that the bad guys are perhaps not as bad as the cops he works for.

Excellent movie, with great performances from Stephen Chiau and Michael Chan. Good action scenes and good direction. Definitely not a comedy, so Chiau fans make sure you know what you're getting!