England 1885. As news reaches England of the defeat of the British army at Khartoum and the death of General Gordon, a group of old war comrades gather at the home of the retired General Faversham. The General's son, Harry, 15 years old, is invited to dine with the group. Already burdened by his father's expectations that he uphold the family's long tradition of military glory, the boy is horrified by the grisly war stories told by the men, and by their united hatred of cowardice. After the dinner, the kindly Doctor Sutton, noticing the boy's discomfort, gives him his card, and tells him to call on him if ever he should need help.

Ten years later, and Harry is now an officer in the army. As it is announced that his regiment is shortly to depart for Egypt to fight against the Sudan uprising, Harry announces his engagement to Ethne Burroughs, daughter of one of his father's military cohorts, and brother of one of his own army friends. Ethne is also loved by Harry's comrade John Durrance, who graciously accepts defeat.

As his regiment is preparing to depart, Harry approaches his commanding officer and tenders his resignation, to the amazement of his friends and fellow officers. Harry is attempting to explain his decision to Ethne, when a package arrives containing three white feathers, one from each of his close friends, Burroughs, Willoughby and Durrance. When Ethne fails to give him the support he hoped for, he adds a fourth feather to symbolise her lack of faith.

Later, during another celebration for departing troops, Harry runs into Dr Sutton, and confides his doubts that his decision may indeed have been motivated by cowardice. He resolves to leave for Egypt alone, and asks Dr Sutton to inform Ethne of his death if he fails to hear from him within a year. In Egypt he seeks out a former colleague of Sutton, and asks for his help. Having heard of a tribe of outcasts, the Sangali, who were punished for rebellion against the Khalifa by being branded and having their tongues removed, he asks the Doctor to help him disguise himself by giving him the brand.

Captain Durrance is commanded to oversee a small rearguard as the main army advances. But having spotted an advance party of the Khalifa's Dervishes, Durrance succumbs to sunstroke and collapses. He is found by his men and returned to the camp. He remains in a coma until the following afternoon, when he awakes to find himself blind. He gives the order to retreat, but at the new camp the company is attacked. Harry, who has managed to reach the camp disguised as a native labourer, is struck down before he is able to raise the alarm. In the battle that follows, the company is defeated, and Burroughs and Willoughby are captured. Durrance is left for dead on the battlefield, where he is found by Harry.

Back in England, Ethne is visited by Dr Sutton, who tells her that since he has not heard from Harry in a year, he must assume he is dead. Meanwhile, braving the desert and its fierce heat, Harry, maintaining his disguise, manages to lead Durrance to the safety of a British outpost, where he returns the white feather, but is caught by British troops assuming him to be a robber and sent to a prison camp.

Six months later, Durrance has returned to England and become engaged to Ethne. At dinner with Ethne, her father and Dr Sutton, he tells the story of his rescue by the mute arab, and when he gets to the 'robbery', produces the only thing he had on his person at the time - a letter from Ethne. When he passes the letter to Ethne, the feather falls out, convincing her that Harry is alive.

Kitchener's army advances towards Omdurman, where Burroughs and Willoughby are still being held captive. Harry, still disguised, manages to speak to them, but is seen and flogged, then thrown into jail alongside them. As the battle rages outside, Harry, unmasked at last, leads a jailbreak and storms the armoury, successfully holding off all attackers until the arrival of the British.

Back in England, news of the glorious victory reaches Durrance, who learns of Harry's heroism and writes to Ethne, telling her, untruthfully, that his sight is to be restored and that he is to rejoin the army. Harry, his bravery now proven beyond doubt, is reunited with Ethne.