August 1914 in the French colony of Gabon in Africa. By the river Ogowe in Lambaréné, Dr Schweitzer runs a small hospital.

It is midnight, and Schweitzer is about to turn in when drumming from tribesmen signals a sick child on its way. The Catholic missionary, Father Charles de Ferrier, arrives carrying a young girl. He reports that tribesmen were preparing to kill her as a devil. Schweitzer takes her for surgery. After he has completed the operation, Schweitzer insists on examining Father Charles, who he thinks is getting thin.

Commandant Lieuvin, the district's military commander, arrives to see Sister Marie, Schweitzer's chief nurse. He proposes marriage and tells her that he will return the next day for her answer.

Leblanc, the civilian governor, arrives by motorboat. He and Lieuvin discuss the attitudes of their people to Africa. Leblanc reminds Lieuvin that, as an Alsatian, Schweitzer is a German citizen, and they may be at war with Germany the next day.

Schweitzer finishes with Father Charles and takes Leblanc for an inventory of the stores. In fact, Leblanc is really there to see Marie. Father Charles and Lieuvin greet each other warmly, having been at military academy together.

Schweitzer returns and announces that the hospital's finances are in a bad state. He plans a tour of organ recitals to raise money but recognises that war may intervene. Lieuvin promises to send further news the next day.

Leblanc reappears, having spoke to Marie, and accuses Lieuvin of having stolen a march on him. Lieuvin leaves.

Leblanc reports that war may be imminent - maybe even that night. He suggests Father Charles stays the night at Lambaréné, thinking he is in danger from the natives, who resent his presence. Father Charles insists on returning to his hermitage an hour's walk away and declines an armed escort. Leblanc leaves, suggesting the barricading of doors.

Later, Father Charles is attacked in his hermitage by tribesmen.

The next evening, Leblanc arrives to see Marie. She is unimpressed with his declaration of love and tells him that she is in love with Lieuvin. He tells her that he and Lieuvin have received sealed orders in case of war.

Schweitzer emerges from theatre. He rejects the notion of a guard on the hospital. Leblanc is offended and departs.

Natives break into and ransack the hospital provisions store. When Schweitzer arrives, they flee. Marie reports that the tribesmen are convinced the hospital is to close.

Lieuvin arrives and tells Schweitzer that war has been declared. They worry about Father Charles. The Africans outside sing a funeral chant in response to the death of one of Schweitzer's patients. Schweitzer goes to say goodbye to the patient.

Lieuvin asks Marie for her answer to his proposal. She says that war changes everything. He tells her that his orders mean he can choose to stay in Africa or return to Europe. He says he has chosen to stay. He reveals he has letters which put him in command over the civil governor, Leblanc. Marie accepts his ring.

Father Charles stumbles into the hospital and collapses, a spear still stuck in his back. He is taken to theatre. Leblanc arrives, having found Father Charles's hermitage pillaged and burnt. Father Charles dies.

Leblanc asks Lieuvin for his decision in response to their orders. Lieuvin tells him he will leave for Europe. He explains Father Charles's death changed his mind. Leblanc tells him he has orders that Schweitzer, as a German citizen, must be arrested before midnight. Lieuvin is shocked and urges him to disobey the order before leaving.

Marie tells Leblanc of Lieuvin's letter giving him supreme power. Leblanc is insulted by Lieuvin's generosity in not using it to send him away. Leblanc tells Marie of Schweitzer's impending arrest and then goes to inform Schweitzer.

Schweitzer takes a final tour of his wards, then asks Leblanc to extinguish the lantern at the landing stage which guides the needy to the hospital. Marie explains she will not join Lieuvin in Europe but will remain at the hospital.

Schweitzer plays piano until Leblanc announces it is midnight and leads him away. As they depart, Marie replaces the lantern by the river.