When news reaches German Eastern Africa that war has broken out across Europe, Rose Sayer and her brother Reverend Samuel Sayer decide to stay at their mission, where they have been spreading the gospel of God for more than a year. During that time, they have seen only one white man - Mr Allnutt, the pilot of the African Queen, a filthy, dilapidated launch that gasses her way up and down the river. But when the German imperial troops descend, they burn down the mission and the Reverend dies from complications, and Rose is left with no option but to leave on the African Queen. But Rose has no plans to escape to safety. Instead she is determined to find a way to do their bit for the British war effort, and avenge her brother's death...