Arriving by seaplane to inspect an isolated but thriving rubber plantation in the African jungle during World War II, Worthing (Richard Ainley) reminisces about the old days, when conditions were much harsher. The film then flashes back to 1910.
The only four white men within hundreds of miles eagerly await the arrival of the riverboat Congo Queen. Wilbur Ashley (Bramwell Fletcher) and his boss, Harry Witzel (Walter Pidgeon), have grown to hate each other. Ashley is finally going home, and the boat is also bringing his replacement, Langford (Richard Carlson), for a four-year stint. The other two white men are the alcoholic doctor (Frank Morgan) and missionary Reverend Dr. Roberts (Henry O'Neill).
Harry and Langford get off to a bad start, and it only goes downhill from there. It takes all of the efforts of the doctor and Roberts to keep the two men from each other's throats. The situation becomes worse when Tondelayo (Hedy Lamarr), a seductive native woman, returns. Harry, as resident magistrate, has already previously ordered her to leave his district, as a disruptive, amoral influence.
Tondelayo begins to work her wiles on Langford. Despite the warnings from all three of the other men (and perhaps to spite Harry), he eventually succumbs to her charms. When Harry orders her expelled once more, Langford decides to marry her. Roberts reveals that she is not a native, but rather half Egyptian and half Arab, and in spite of his better judgment, reluctantly joins them in holy matrimony.
After five months, Tondelayo has grown bored of her husband. However, when she tries to seduce Harry, he reminds her that she is Mrs. Langford "until death do you part". That gives her an idea. When her husband becomes sick, the doctor gives her some medicine to give him periodically. She obtains poison and makes him drink some of it instead. However, Harry suspects what she is trying to do. He leaves, then returns just as she is about to give Langford another dose. Harry forces her to drink the rest of the poison. She runs away screaming and collapses on the jungle floor.
The doctor takes Langford away on the Congo Queen for better medical treatment, identifying him as white cargo. From the boat comes Langford's replacement: a younger Worthing. Harry grabs him and forcefully tells him that he will stick around. Returning to the present, Worthing observes that he did.
