British colonial rulers are plundering the country for all its worth, treating the locals unjustly and causing widespread resentment. Worst of all they are using the Indians as sepoys, the infantry of the army, to control and regulate the population. After a hundred years of subjugation, India is awakening to the revolutionary prospect of change and self-rule.

During a fierce battle in the Afghan wars of the mid-century, Mangal (Aamir Khan), a heroic sepoy rescues his British commanding officer William Gordon (Toby Stephens). The event creates a strong friendship and binding loyalty between them that transcends consideration of rank and race. The friendship is soon challenged, first by arrival of a charming and beautiful young aristocrat, Emily Kent (Coral Beed), and then by the introduction of controversial new gun cartridges among the troops.

The relationship is complicated further when Gordon saves a beautiful young native girl, Jwala (Amisha Patel), from the funeral pyre of her late husband, and falls in love with her. The new cartridges require soldiers to bite through their greased casing, made of animal fat, and suspicion spreads that the British are ignoring religious beliefs in favor of cheap weapons, and polluting their Muslim and Hindu soldiers.

Gordon assures Mangal that the cartridges are free from pollution and demonstrating his total trust in Gordon, Mangal bites the cartridge. They soon discover that it really is animal fat and the rumor of this imposed pollution is the spark that ignites the powder keg of resentment in the country. Mutiny breaks out with Mangal growing in stature to lead the Indian people to freedom.