In turn-of-the-century London, the British government fears that Russian interference may be creating a dangerous situation at the Afghan-Indian border and orders the Royal Fusiliers to advance to the area. Although worried about the dangers, regiment commander Col. Lenbridge wants his son Dion, now a captain at the war office, to transfer back to the regiment before they sail. Dion is happy to rejoin his father and hopes to prove to the troop that he is no longer just "the colonel's little boy." Dion's cartographer, Herbert Bielensen, offers his best wishes, just as Maj. Mac Street, Dion's replacement, takes possession of Dion's dispatch box. Later, while Dion has lunch with his sweetheart, Jane Wensley, Bielensen secretly goes to the Russian embassy to tell the Emissary that the dispatch box, to which Bielensen was to return some stolen documents, is now inaccessible and their theft will soon be discovered. The next morning, as Dion is packing to leave for India, he pays off his bookie and starts to relate to his friend and rival for Jane's affections, Capt. Thomas Garron, that he has just won a large sum of money playing cards with a complete stranger. Just then Bielensen arrives with the gift of a cake, and Dion hurriedly leaves for his ship after giving his aide a large sum of money to settle his many debts. At the dock, before boarding, Dion says goodbye to Jane, who is soon to join her father, Maj. Wensley, in India, but as they happily talk of their impending marriage, a contingent of soldiers interrupt them and arrest Dion. At a court-martial, it is revealed that when Mac Street opened the dispatch box, he found two important documents missing. Later, one of them was found among Dion's things. Dion is not worried, despite the additional evidence of his unsubstantiated financial windfall, because he is sure that Bielensen's testimony will prove his innocence. Although Bielensen appears to be a reluctant witness, his testimony that Dion took both documents and put them in his pocket is damning. The military court sentences Dion to be reduced to the ranks and drummed out of the army. After his humiliation before the regiment, Dion is arrested by civilian authorities and charged with treason. While awaiting trial, Dion refuses to see the loyal Jane and tells his attorney, Maj. Fallow, that he will somehow find Bielensen and make him talk. Later, as the police escort Dion back to jail, he escapes. Now fearful for his life, Bielensen returns to the coastal town where he lives and asks for help from his Russian cohorts. A short time later, Dion arrives in the town looking for Bielensen, but learns that he went out fishing one day and never returned. Dion then buys some old clothes and throws his own into the sea to make it appear he has drowned. Some time later, after a series of menial jobs, Dion decides to enlist as a private in the army, assuming the name "Harry Simms." Dion pretends not to adapt well to military life and is viewed with antagonism by many of his fellow recruits, who call him a "gentleman ranker." His only friend is Pvt. McGinty, a kindhearted man who has difficulty learning military techniques. Dion helps McGinty, and the two men pass their tests and sail toward India with their recently called up regiment. Months later, the regiment is ordered to a base near the Khyber Pass, as reinforcements for the Royal Fusiliers. Jane, who is now in India and being romanced by Tommy, still loves Dion, even though it was reported that he drowned. One night, Dion secretly approaches her and she is overjoyed that he is still alive. Although Dion tries to avoid contact with his former regiment, his identity is soon uncovered and he is arrested. Jane tries to persuade Lenbridge that his son is innocent, but Lenbridge feels that the military court could not have been wrong. Soon Lenbridge receives word that the Afghan leader, Hassan Khan, has been fomenting trouble in the Khyber Pass. Tommy suggests disguising himself as an Afghan to go through the pass and survey the situation, and Lenbridge agrees. That same night, McGinty, who believes in Dion's innocence, is assigned to guard him. While the two men enjoy a beer that McGinty has brought, they hear shooting close by. Fearing that his defenseless friend will be slaughtered if the camp is attacked, McGinty allows Dion to escape. Dion then wanders the surrounding mountains and shoots at Tommy, who he assumes is an Afghan. The two men scuffle, but after they recognize each other, Dion binds Tommy's wounded arm. The two men join forces and soon come upon the bodies of many dead British soldiers. Although Tommy is getting weak, Dion helps him to the next British encampment, where they report their findings. None of the soldiers recognize Dion at this regiment, and he is hailed as a hero. When some of the troops are ordered to go back through the Khyber Pass, Tommy urges Dion to escape, but he refuses. The regiment's colonel, who admires Dion's bravery and skills, assigns him to be orderly to a green young officer named Lt. Jersey. The Afghans, stirred into action by Hassan Khan and the Russian emissary, who has joined them, soon ambush the British, and casualties are heavy. The wounded colonel puts Jersey in charge and, following the colonel's advice, allows Dion to take a small contingency of men to sneak behind the lines. Dion sees the enemy's movements and relays valuable information that is then signaled back and forth among Jersey's men and two other divisions. On Dion's advice, Jersey advances his men at just the right time and the British are victorious. "Simms" is told he will be recommended for a decoration as the men march out to meet the oncoming Royal Fusiliers. Back at Lenbridge's headquarters, the Russian emissary is brought before him. Hoping for leniency, the emissary confesses his part in the theft of the documents in London and attempt to frame Dion. Lenbridge then happily tells his son that he has been exonerated, and Jane and Dion are married.
