The year is 1811. With the British army penned into a small part of Portugal, and all of Spain fallen to the invader except for the coastal city of Cádiz, the French appear to have won their war. Captain Richard Sharpe has no business being in Cádiz, but when an attack on a French-held bridge goes disastrously wrong, Sharpe — accompanied by Harper, his loyal Irish sergeant, and the obnoxious Brigadier Moon — finds himself in a city under French siege.

It is also a town riven by political rivalry. Some Spaniards believe their country's future would be best served if they broke their alliance with Britain and forged a friendship with Napoléon's France; their cause is only strengthened when some letters written to a prostitute by the British ambassador fall into their possession. They resort to blackmail, and Sharpe, raised in the gutters of London and taught to fight, is released into the alleys of Cádiz to find the woman and retrieve the letters.

Yet defeating the blackmailers will not save the city. That is up to the charismatic Scotsman, Sir Thomas Graham, who takes a small British force o attack the French siege lines. The attack goes horribly wrong; Sir Thomas's outnumbered army is trapped between the devil and the deep blue sea, and on a March morning, at Barrosa, Richard Sharpe finds himself embroiled in one of the most desperate infantry struggles ever fought. Sir Thomas has his own reasons for revenge, as does Sharpe, who goes into battle seeking the French colonel who precipitated the disaster that stranded Sharpe in Cádiz. In a bloody and stirring battle, Sharpe and the English get their revenge and their victory, but at a terrible cost.