General Wellington gives Richard Sharpe a crucial secret mission: to retrieve a hoard of Spanish gold he desperately needs to continue fighting the French.

The gold is behind enemy lines, in the keeping of Spanish guerrillas led by El Católico, who delights in torturing French prisoners to death.

Claud Hardy, one of Major Michael Hogan's exploring officers, was assigned to keep an eye on the guerrillas and the gold after a failed attempt by British cavalry to fetch it.

When Sharpe meets El Católico, the latter admits knowledge of the gold and strongly implies the British intend to take possession of it rather than merely escort it to the Spanish government in Cadiz.

Sharpe and his men are caught on open ground, about to be overrun by French lancers, but are rescued by a unit of King's German Legion cavalry under Captain Lossow, who was sent by Hogan to search for them.

Unable to contact Wellington, as the telegraph is destroyed by French artillery before a message can be sent, Cox orders Sharpe to surrender the gold to El Católico and join the garrison.

When Sharpe delivers the gold (less what he gives Teresa and keeps for himself and his men), he learns that it is to pay for the construction of the enormous defensive lines of Torres Vedras, which form an impregnable barrier between Marshal Masséna's army and Lisbon.

Sharpe is granted a month's leave by Wellington, so he takes the opportunity to renew his acquaintance with Josefina LaCosta, who has set herself up in Lisbon as an exclusive courtesan.

[2] Although El Catolico and his treasure trove are literary inventions, the guerrillas and gold alluded to in this novel were an important part of the war against France ("the twin allies of British victory"); Cornwell admits that the "Sharpe books do not do justice to the guerillas".

[2] The books tells a fictionalised account of the destruction of Almeida which, as Cornwell notes "conveniently for a writer of fiction",[3] remains a mystery.

A 1995 TV adaptation of the same name was produced by Central Independent Television for the ITV network in the UK starring Sean Bean and Daragh O'Malley although this bore little resemblance to the novel.