When a prim missionary and a rough-hewn mechanic find themselves stranded in German East Africa at the outbreak of World War I, they must navigate a treacherous river to strike a blow against the enemy. Rose Sayer and Charlie Allnutt are forced into an unlikely alliance after the death of Rose's brother leaves her alone in a hostile territory. Their objective is to pilot the small, dilapidated steam launch, the African Queen, down the Ulanga River to attack a German gunboat. They face extreme physical obstacles including rapids, disease, and the constant threat of discovery by German forces. The narrative follows their evolving relationship as they transition from mutual disdain to a shared, desperate purpose.

Readers frequently highlight the stark contrast between the two protagonists as the primary driver of the narrative tension. Critics often point to the meticulous detail regarding the boat's operation as a grounding element for the more improbable survival sequences. Discussion often centers on the transformation of the characters as they shed their societal expectations in the face of isolation. The pacing is noted for its steady escalation, mirroring the dangerous descent of the river itself. Many readers appreciate the lack of sentimentality in the prose, which maintains a focus on the logistical and physical reality of the characters' situation.