Sharpe's Devil is a novel about the ongoing adventures of Lieutenant Richard Sharpe. In this adventure, Sharpe is hired by a wealth Spanish countess to go to Chile and find her missing husband. Along the way, Sharpe must negotiate obstacles presented by corrupt Spanish officials, rebel fighters, and even Napoleon Bonaparte.

Since leaving military service, Richard Sharpe has lived with his companion Lucille and their two children on a farm in Normandy. Sharpe's calm and rustic life is interrupted by a visit from the Countess of Mouromorto. Sharpe first met this woman many years previously when she was simply Louisa Parker, a young English woman fleeing the warfare in northern Spain. Louisa met and married Don Blas Vivar, Count of Mouromorto. Sharpe is a friend of both Louisa and her husband, and Don Blas once helped Sharpe during the war. Louisa has come to reclaim the favor.

To stop the Chilean rebellion, the government of Spain sent Don Blas to Chile. Sometime after his arrival, Don Blas disappeared, and no one, not even the Spanish government, seems to know what happened to him. Louisa believes he is still alive, and she hires Sharpe to go find her husband.

After accepting the job, the first thing Sharpe does is to contact his friend Patrick Harper. Harper was Sharpe's Regimental Sergeant during the French Wars, and the two men are close friends. Since leaving military service, Harper has been operating a pub in Dublin with his wife and children.

Sharpe and Harper appear as an odd pair to outsiders. Sharpe is tall, thin, and battle scarred. His personality tends to be a bit stern. Harper is hugely obese and jovial.

The two men get passage to Chile on a Spanish frigate, the Espiritu Santo. Along the way, they stop at the island of Saint Helena where Sharpe meets Napoleon Bonaparte. Napoleon asks Sharpe if he will deliver a gift to a friend in Chile. Sharpe agrees. Napoleon gives Sharpe a portrait to deliver.

After arriving in Chile, Sharpe finds that the local authorities are not at all cooperative in his quest to find the missing Don Blas. Everywhere Sharpe looks he encounters resistance and cryptic information. Not long after arriving in Chile, all of Sharpe's and Harper's belonging are stolen. The belongings are recovered and returned to Sharpe and Harper by the sadistic commander of Spanish Chile, Captain-General Miguel Bautista. One possession remains missing: the portrait of Napoleon.

Captain-General Bautista later finds the portrait and discovers that a coded message is hidden inside. He uses this as a reason to banish Sharpe and Harper from Chile. On the sailing trip back to Europe, the ship Sharpe and Harper are on is captured by the Chilean rebels led by the charismatic and daring Thomas Cochrane. Sharpe and Harper join with Cochrane and fight several bloody battles alongside the Chilean rebels.

After helping Cochrane capture the last remaining Spanish stronghold in Chile, Sharpe is surprised to discover that he still cannot find Don Blas. He is even more surprised to discover that Cochrane has been misleading him all along. The likable and brilliant trickster Cochrane has been in control of everything since the beginning, and he has a succession of surprises in store for Sharpe until the very end of the novel.