In Peshawar, capital of the Northwest Province of India, Canadian Kyle Cameron, a captain in Britain's Eleventh Lancers Regiment, is reprimanded by his commanding officers, Col. Rivington and Gen. Melville. The headstrong Cameron has offended Prince Ishak Khan, an Afghan revolutionary under house arrest. Melville explains the necessity of maintaining diplomatic relations with Khan, as he is concerned that Russia may arm Khan and the independent hill tribes on the border of Afghanistan and India in order to gain control of the Khyber Pass, and orders Cameron to apologize formally. Cameron also falls afoul of Melville's daughter Diana, with whom he was once romantically involved but who is now enamoured of Lt. George Kennerly. After Cameron visits Khan to apologize, Khan receives a coded message that his people are ready to seize the Pass and that Russian aid is assured. When Cameron is assigned to round up the attackers of a caravan in the Pass, he is warned not to cross the Afghanistan border. However, exactly at the border, Cameron and his troop come under fire from snipers on the Afghan side, and he crosses over to retaliate. There, he encounters a group from the Afghan Border Patrol led by an old friend, Ahmed, who allows them to leave. Back at headquarters, Rivington disciplines Cameron, who then becomes involved in a fistfight with Kennerly, who is about to become engaged to Diana. Later, all the officers are shown a new secret weapon, an automatic machine gun that will fire six hundred rounds a minute. As Cameron is already familiar with an American version of the gun, he is ordered to instruct the others in its use. Ahmed informs Cameron that a coded message to Khan indicates a meeting of five tribes in a nearby village to plan a raid. Cameron decides not to report this to Rivington and summons Kennerly and his troop to go along on the mission. Unknown to Cameron, Ahmed is actually working for Khan and sends the soldiers into an ambush, during which many lives, including Kennerly's, are lost. Cameron's unofficial actions result in his being brought before a board of inquiry, where Rivington voices his suspicion that Cameron may have deliberately caused Kennerly's death. At the hearing's conclusion, Rivington requests Cameron's resignation from the regiment, but he refuses and is relieved of duty. Cameron tries to explain Kennerly's death to Diana and she accepts that he was not immediately responsible. Believing that Ahmed betrayed him, Cameron is able to prove Ahmed's alliance with Khan and reports this to Melville, who asks him to commit another offense and be thrown out of the regiment in order to work as an undercover spy. Cameron realizes that he may never be able to clear his name, but is willing to risk the disgrace in order to infiltrate Khan's forces. Melville is particularly concerned that Khan is planning to attack the train bringing more machine guns. Cameron initiates an argument with Rivington, strikes him, then deserts and is recruited by Ahmed to join Khan. Cameron learns the location of the train attack and relays the information to Melville via a harem girl, who is a British spy. With Khan concentrating on the railway, Melville decides to ship the guns by wagon. After Khan takes Diana hostage as protection that Cameron will not betray him, he learns of the change in plans and attacks the wagon. Cameron manages to free Diana and comes to the rescue of his former regiment by assembling one of the machine guns and shooting Khan, Ahmed and their followers. Later, Cameron and Diana leave on a honeymoon.