September 1914, news reaches the colony in German East Africa that war has broken out across Europe making British Reverend Samuel Sayer a hostile foreigner. German imperial troops burn down his mission; he is beaten and dies of fever. His well-educated, snobbish sister Rose Sayer buries him and leaves by the only available transport, the dilapidated river steamboat 'African Queen' of grumpy Charlie Allnut. As if a long difficult journey without any comfort weren't bad enough for such odd companions, she is determined to find a way to do their part for the British war effort while avenging her brother and aims high, as God is obviously on their side: construct their own torpedo and use the converted steamboat to take out a huge German warship, the Louisa, which is hard to find or reach on the giant lake. She presses till Charlie accepts to steam up the Ulanga, brave a German fort, raging rapids, bloodthirsty parasites, and the endlessly branching river which seems to go nowhere but impenetrable swamps. Despite fierce rows and moral antagonism between a bossy devout abstentionist and a free-spirited libertine drunken loner, the two grow closer to each-other as their quest drags on.