It is based on the book Heart of darkness, a 29th century classic by Joseph Conrad, it is a book studdied on Literature courses at Degree level, it was included in one of the modules I did at Uni.
a US colonel has gone upriver during the Vietnam war and gone native; a man is sent after him to kill him. The original book dealt with Belgium and the Belgian Congo as was, but the story adapts pretty well to the 1970s.