During a general British and Portuguese retreat from the French after the First Battle of Porto, Captain Hogan orders Lieutenant Richard Sharpe and his men to help find and escort to safety 19-year-old Englishwoman Kate Savage, the daughter of a recently deceased prominent port merchant.

Hogan also tells Sharpe to "keep a close eye" on Colonel James Christopher, who has been staying with the Savages and was the one who requested help in retrieving her.

Sharpe is suspicious of his motives, but Christopher shows him his orders from General Cradock, the commander of the British forces in southern Portugal.

Christopher then negotiates with the French, offering the identities of the mutineers and other information in return for a monopoly on the port trade in Oporto.

In the aftermath of the British victory and the French retreat, Sharpe informs Wellesley and Foreign Office dignitary Lord Pumphrey of Christopher's treason.

The French manage to capture both bottleneck bridges from Portuguese irregulars and escape, but Sharpe finds and kills Christopher (and one of his riflemen who deserted) and rescues Kate.