During World War I, a strait-laced missionary, Rose Sayer, finds herself stranded in East Africa after her brother's death. She enlists the help of Charlie Allnut, a coarse, gin-drinking boat captain, to escape by navigating his dilapidated steamboat through treacherous jungle rivers. Their journey is fraught with challenges, including dangerous rapids, insects, and a German gunboat patrolling the river. Despite their stark differences and initial animosity, Rose and Charlie must learn to work together to survive the perilous journey.

As they brave the elements and external threats, Rose devises an audacious plan to strike back at the enemy by using Charlie's boat as an impromptu weapon. Their adventures lead to both moments of intense conflict and unexpected camaraderie. Faced with mechanical failures, environmental hazards, and enemy forces, the duo's relationship evolves amidst the relentless trials. Together, they become a formidable and unorthodox team, finding determination and resilience in each other's company as they pursue their desperate mission against the odds.