Robert Morley and Katharine Hepburn play Samuel and Rose Sayer, brother and sister British Methodist missionaries in the village of Kungdu in German East Africa at the beginning of World War I in August/September 1914. Their mail and supplies are delivered by the rough-and-ready Canadian boat captain Charlie Allnut (Humphrey Bogart) of the African Queen, whose coarse behaviour they tolerate in a rather stiff manner.