West Africa under British rule. The King's Commissioner Sanders rules the River Territories with firm but just efficiency. Bosambo, who has appointed himself chief of the Ochori tribe, visits Sanders to report that the warriors of King Mofalaba have been carrying out slave raids on the tribes of the river. Sanders chides Bosambo for assuming power without permission, but in return for his information, he appoints him chief of the Ochori.

Sanders receives news that Mofalaba's warriors are advancing on the Ochori. Sanders sends notice to Bosambo, who gathers his own warriors and defeats Mafalaba's forces, freeing their slaves. One of the slaves, a woman called Lilongo, attracts Bosambo's attention.

Sanders summons Mafalaba to palaver, but Mafalaba defiantly brings his entire army. Sanders chastises Mafalaba and warns him to abandon slavery or be deposed. Mafalaba, who fears Sanders, agrees, and is dismissed, but not before threatening revenge on Bosambo. Later, Bosambo and Lilongo are married, with Sanders blessing.

Five years of peace and prosperity follow for the people of the river, particularly for the Ochori, under the chieftainship of Bosambo, who now has a young son. Sanders prepares to take a year's leave to be married, and hands over the reins of power to his deputy, Ferguson. Before he leaves, he warns his successor to be wary of King Mofalaba, suggesting that he invite him to palaver and make a show of force. But Ferguson prefers to meet the other chiefs of the river, and Sanders arranges this.

With Sanders gone, two unscrupulous white traders, Farini and Smith, take advantage of his absence to sell guns and gin to the natives, and spread a rumour that Sanders is dead, and the law with him. Within a week, the land has descended into anarchy. Ferguson, uncertain how to respond, finally decides to enter Mofalaba's country alone. But he lacks Sanders' status, and he is killed.

Meanwhile, Sanders receives news of the uprising and returns by aeroplane. Bosambo spreads the news of Sanders return, which reaches King Mofalaba, who has Farini and Smith for their lies. Reckoning that Sanders will not come until the Spring, Mofalaba despatches warriors to kidnap Bosambo's wife, hoping to lure Bosambo into a trap.

Sanders instructs Bosambo to exact his punishment of the other warring tribes of the river, but on returning to his own tribe, Bosambo discovers Lilongo's disappearance and sends his children with a message to Sanders before heading for Mofalaba's country alone. Sanders, recovering from Malaria, head upriver after Bosambo.

Bosambo enters Mofalaba's country and his captured, but before he can be executed, Sanders boat arrives and lays waste to Mofalaba's warriors with machine guns. Sanders confronts Mofalaba, defends himself, claiming that Farini, not he, killed Ferguson. When Mofalaba raises his spear against Sanders, he is killed by Bosambo. Sanders appoints Bosambo as King of the lower river.