Sharpe's Battle is a detailed description of Richard Sharpe's life for a few days in May 1811. Sharpe is a rifleman officer in the British army fighting Napoleon's forces. Sharpe manages to enrage a French general who vows to get him and gets caught in politics that could ruin his career. Sharpe is given charge of the Real Compañía Irlandesa, the guard for the Spanish king, and he is determined to turn them into soldiers against his superiors' desires. As thousands of French converge on a tiny village, Sharpe's hope for stopping the court of inquiry and restoring his honor depends on leading these men to special glory on the battlefield.

Richard Sharpe and a group of his riflemen stumble onto a village that the French brigade under the command of General Guy Loup has ransacked, murdering all but one woman. The pile of dead children, stacked like animals, and catching two of the French in the act of raping a young woman enrage Sharpe. He takes the two French prisoner, determined to execute them for their crimes. Loup, their commander, rides up under a flag of truce and asks for the two men. Sharpe sets up a firing squad and kills them in front of Loup. Loup leaves vowing he will find and kill Sharpe.

At the British base, Major Michael Hogan informs Wellington that the King of Spain has sent his guard, the Real Compañía Irlandesa to join the British in the fight against the French. Both men see it as a set-up and believe that the Real Compañía Irlandesa's real mission is to sabotage the British efforts. They place Sharpe in charge of the guard and send them off to San Isidor, a deserted crumbling fortress of no tactical value.

Pierre Ducos, a French intelligence officer, and Juanita de Elia wait for Guy Loup at the Ciudad Rodrigo. He and de Elia will be working to undermine British troops by attempting to incite the Irish to rebellion. Their means will be false newspaper articles delineating British atrocities against the Irish people at home.

Wellington also sends a Portuguese battalion to San Isidoro to keep an eye on the Real Compañía Irlandesa. Though Sharpe warns the Portuguese commander that he believes Loup will attack them, the commander does not heed his words as well as he should. The night the Portuguese arrive, Loup attacks and though most of the Real Compañía Irlandesa men survive three quarters of the Portuguese are slaughtered.

Hogan appears at San Isidoro and informs Sharpe that Runciman, the liaison between the Real Compañía Irlandesa and the British will be sacrificed to a court of inquiry to satisfy the Spanish and the Portuguese. Later, Sharpe is also called to task for shooting the two prisoners and inciting Loup to a personal vendetta. He too will be sacrificed for political expediency.

Sharpe is put on administrative duties with the ammunition depot and he spends much time trying to get into the fighting in order to save himself from the court of inquiry. As the French are within a short distance of overrunning the British position and thoroughly annihilating them, Sharpe receives permission via Runciman, to take the Real Compañía Irlandesa and counterattack the French. They are able to push the French back and Sharpe wins a hand to hand combat with Loup resulting in Loup's death. The court of inquiry is cancelled and Sharpe and Runciman's careers are saved, as is the British position in Spain.