At the headquarters of the 8th Regiment Hussars, deep in the heart of colonial Africa, Commissioner 'Sandy' Saunders is visited by tribal chief Mosambo, requesting champagne to celebrate the return of his brother, M'Bapi. The plea is refused, as the British deem alcohol illegal for the natives. Mosambo accepts the judgment with no ill will.
M'Bapi is returning to his tribe after completing his education at Oxford, but he plans to win over his people with illegal gin and a return to independence. He further intends to gain control of their land and sell mining rights to crooked foreign investors. On the ship to Africa he meets Benjamin Tibbetts, an English teacher who has grandiose plans to educate Africans and open a string of schools across their continent. M'Bapi cons him into taking his vast supply of alcohol ashore, aware that as Tibbetts is on official duty, his possessions will not be searched. The plan works, with Tibbetts unwittingly delivering M'Bapi's tools for a revolution.
Saunders departs the H.Q. on leave, with Captain Hamilton placed in charge during his absence. His first task is convincing Tibbetts to begin his overly ambitious project on a small scale, teaching a class of the local children. The teacher agrees, but due to his incompetence and the natives' incredulity at his eccentric methods, the lesson degenerates into a shambles.
Meanwhile, Hamilton has fallen seriously ill with a fever and when he is sent away to visit a doctor, Tibbetts takes charge of the Captain's duties in the absence of Saunders. The schoolteacher's pomposity swells with his newfound status and he resolves to travel down the river to collect overdue taxes from local tribes.
En route he encounters the S.S. Zaire and its crew, which comprises the elderly Harbottle and his younger subordinate, Albert. Both men are unprincipled idlers who customarily work for Saunders, helping in the collection of taxes. Tibbetts commandeers their ship and their assistance, although the tribes people of the first village quickly recognise the schoolmaster's ineffectiveness and in lieu of taxes leave a herd of goats on the ramshackle Zaire.
Further down the river, using strong gin and false rhetoric, M'Bapi has usurped his brother and become leader of his tribe. The new chief promises might and revolution, but when Tibbetts arrives he realises Hamilton's deputy must leave suspecting nothing is awry, or be killed to ensure he does not warn the captain of the intended uprising. Under pressure, Mosambo plays out a charade of normality and, after the taxes have been collected, the Zaire departs. However, Mosambo escapes, catches up with Tibbetts and recounts recent events, adding that his brother is planning a human sacrifice in accordance with his tribe's ancient religion. Realising he is culpable as he delivered the alcohol ashore, Tibbetts returns to the village and, through bravery and good fortune, is able to rescue the baby due to be ceremoniously executed. Together with Albert and Harbottle, he sets sail for the garrison.
However, their warning message to H.Q. is badly phrased and misunderstood, resulting in the garrison's soldiers immediately being sent on leave. When the trio arrive back at the base they realise they must defend it alone against M'Bapi's attacking forces until the troops return. Through desperate trickery they create the illusion that the complex remains guarded by a full set of soldiers, but undeterred, the tribesmen breach the outer walls. Tibbetts, Harbottle and Albert now delay their progress using barrels of sharp metal tacks which further hinder and injure the barefoot natives. Mosambo arrives with the soldiers, kills his treacherous brother and the base, it seems, is once more secure.
The schoolmaster is warmly congratulated by the returning Saunders, whose platitudes are cut short when some dynamite, which had earlier been prepared, primed and forgotten, explodes. A frazzled Tibbetts is left to rue his oversight.
