An English spinster named Rose finds herself in a dire situation after her brother, Rev. Samuel Sayer, a Christian missionary, takes his own life following a brutal attack on their remote African village during World War I. The village, situated beside the Ulanga River in a German colony, is ravaged as the German military sets it aflame, causing immense devastation. Left alone and grappling with despair and imminent danger from the Germans, Rose contemplates her fate amidst the ruins around her.
Just as hope seems lost, a scruffy boat captain named Charlie Allnut arrives to deliver mail and supplies. This rugged man, who operates a rickety old boat called the African Queen, becomes Rose's unlikely ally. After burying her brother, Charlie takes her aboard the African Queen, and the two begin a tumultuous journey down the Ulanga River towards Lake Victoria in search of freedom. Despite their deep differences—Rose's determination to face the German gunship Louisa, for instance, clashes with Charlie's cynical view of the situation—there emerges a bond between them as they navigate personal challenges and external threats.
While they initially drive each other to frustration, with Rose taking a stand against Charlie’s excessive drinking and Charlie feeling plagued by her moral standards, their experiences force them to rely on one another. As they confront treacherous waters and meager conditions, Rose's admiration for Charlie's seamanship grows amid the thrill of their shared adventures. During dangerous encounters with the river’s rapids and enemy fire, sparks fly, leading to moments of tenderness and burgeoning love, evident in their intimate moments and support for one another.
As they reach the mouth of the river, their true challenge stands before them: to devise a plan to sink the Louisa. They ingeniously prepare homemade torpedoes, determined to transform the African Queen into an unexpected threat to the German warship. However, as they edge closer to their daring confrontation, nature intervenes with a violent storm that capsizes their boat, separating them and leaving their fates uncertain.
In a twist of fate, Charlie is captured by the Germans, while Rose escapes unharmed. In captivity, he remains despondent, convinced that he has lost Rose forever until, unexpectedly, her voice reaches him from a lifeboat nearby. Desperate to protect her, he decides to deny their connection, hoping to shield her from the brunt of German wrath. Yet, undeterred, Rose boldly recounts their incredible escapades to the German captain, further sealing their unique bond.
As the two confront their impending execution, Charlie proposes a marriage amidst the chaos—a whimsical yet heartfelt request that surprises even the indifferent German captain. Just as they are united as husband and wife, the unforeseen happens: the African Queen, having survived against all odds, inadvertently collides with the Louisa, leading to an explosive climax that signifies both their liberation and their shared triumph over adversity.
With the two lovers now navigating the water together, they embrace their unexpected survival, singing joyfully as they drift toward safety, hopeful for a future where love and resilience prevail against all challenges.
