Set in the summer of 1812 including the Battle of Salamanca on 22 July 1812, the story follows Sharpe and his friend Sergeant Harper involved in espionage while hunting down the sadistic and highly dangerous Colonel Philippe Leroux.

French Colonel Philippe Leroux and Captain Paul Delmas are fleeing from the King's German Legion toward Sharpe's Light Company.

Leroux has extracted the secret identity of El Mirador, Britain's most important spy in Spain, from a priest he tortured.

Leroux kills Delmas and assumes his identity and then allows himself to be captured by Sharpe and his men, knowing that the British would never exchange an imperial colonel.

In Salamanca, Sharpe is introduced to the breathtakingly beautiful Marquesa de Casares el Grande y Melida Sadaba, and to Captain Lord Jack Spears.

Leaving his jacket behind (he had taken it off due to the heat), Sharpe and Harper race to the hospital, disrupting Leroux's rendezvous with a confederate who has brought a horse for him.

The Light Company eventually realise that Sharpe and Harper are missing and Major Hogan is alerted.

When his discarded trousers are found, it is believed that he was mistaken for a dead French soldier and buried in a mass grave.

A month later, Hogan sends Sharpe a letter telling him that the French will soon be returning to Salamanca and that he must pack and leave.

Curtis tells Sharpe that one of his correspondents in Paris has discovered that Leroux has a multi-lingual sister named Hélène.

Hogan does as well, and asks Sharpe to feed her false information that Wellington intends to speedily retreat to Portugal, while remaining with one division as rearguard to fool the French into believing otherwise.

Sharpe, still not fully healed, rejoins Wellington's army, riding on a horse that was a gift from La Marquesa.

Sharpe seeing the South Essex being pushed back and realising that they need to stand firm to channel the French columns into a killing ground for the Sixth Division, cannot resist joining the battle.

The French withdraw under the protection of their still undefeated right, hoping to cross the bridge at Alba de Tormes and escape.

Sharpe realises that he is lying and suspects that he is the traitor in the British headquarters who stole Hogan's list.

La Marquesa is allowed to leave Salamanca, since it is not in the British interest to create a scandal involving a high-ranking Spanish aristocrat.

Sharpe chooses to keep Harper's present, feeling it is lucky, while throwing Leroux's sword – which, despite its beauty, has only been used for evil purposes – into the river.

Lieutenant General Wellington, Marshal of France Auguste Marmont, Patrick Curtis, Sergeant Connelley (in charge of the death ward in the novel) and Colquhoun Grant (exploring officer captured by Colonel Leroux) were all based on real historical figures of the same name, with limited dramatic licence taken.