A small settlement in German East Africa is destroyed by a German attack during the First World War. Pastor Samuel Sayer, who has been running a mission together with his reserved sister Rose, is killed in the attack. Rose is now dependent on the help of rough-and-ready river captain Charlie Allnutt, who is to take her to an inhabited area in his ship, the African Queen. During the journey, a conflict-ridden relationship develops between the two very different characters. However, the confrontations bind them together more closely instead of driving them apart. Thus, Charlie and Rose finally hatch a plan to punish the German troops that destroyed the village. But the dangers of nature make the undertaking seem almost impossible. The difficulties they face along the way are almost insurmountable. A German position, which they take under fire, rapids that are considered impassable, and a labyrinth of shallow branches in the estuary, full of reeds, mosquitoes and leeches, where Charlie has to drag the boat by hand to the point of exhaustion. Both are captured by German officers and sentenced to death by hanging. During the execution, the ship they are on capsizes and sinks. Rose and Charlie reach British territory unharmed.