Twice Under (1989)

Cannel expert Ed is asked for advice by the local police in search for the murderer of two sewer-men. Soon Ed realizes that not he's seeking the murderer, but the other way around: it's his former friend Daggat, who's come for revenge for an incident in Vietnam, where Ed would have let him die. His half-Vietnamese teenage son Rick fears that Ed has no chance against Daggat and hurries after him to help or at least take counter-revenge.