Based on C. S. Forester’s novel, a man and a woman go on his boat down an African river to attack a German ship during World War I.
In German East Africa in September 1914 Rev. Samuel Sayer (Robert Morley) tries to lead Africans in singing a hymn as Rose Sayer (Katharine Hepburn) plays the organ. Charlie Allnut (Humphrey Bogart) arrives with their mail. Rose serves tea while Allnut’s stomach growls. He says Germany and England are at war, and he leaves. German soldiers knock down Samuel and set the village on fire. Samuel is delirious and dies. Allnut comes back and digs a grave. Rose packs and goes with Allnut on his boat.
Allnut says the Germans have a fort and the Louisa on the lake. Rose asks Allnut if he could make a torpedo. They get started, and Allnut drinks gin. They bathe in the river. Rose lets him come in out of the rain. They go down river and through rapids. Rose is thrilled and wants to go on.
Allnut gets drunk, says he won’t attack the Louisa, and calls her a “skinny old maid.” Rose pours out all the alcohol. Allnut shaves and has cleaned the engine. Rose reads the Bible, and Allnut apologizes. Rose says he promised. They go on.
As they pass the Shona Fort, soldiers shoot at them and damage the engine. They go through rough rapids. They embrace, and he kisses her. They realize they are in love and are nice to each other. They enjoy the flowers, and he imitates the animals. Falls damage the boat. Charlie dives under the boat and says the propeller is broken. They remove it and repair it with welding.
The river is calm. They are attacked by insects, and Charlie covers her and gets the boat moving. They get lost in the delta grass. Charlie in the water pulls the boat and gets leeches. Rose uses salt to remove them. They realize they are stuck in the mud, and she prays.
Rain raises the river, and the boat floats into the lake. Charlie wakes and tells Rose. They see the Louisa and go back to the reeds to hide. Charlie makes detonators, and they put explosives in two oxygen cylinders. Rose insists on going with him.
At night they go to attack the Louisa, but in a storm they sink. The Louisa Captain (Peter Bull) questions Charlie and accuses him of being a spy; he is sentenced to be hanged. Rose is brought on board and questioned. She says they were going to attack them. Charlie asks the Captain to marry them. After that, nooses are put around their necks. The Louisa hits the torpedoes on the African Queen and is sunk. Mr. and Mrs. Allnut swim toward the east shore.
This romantic adventure brings together two contrasting characters through the dangers they endure by working together. They survive and risk their lives fighting the German enemies of their countries.