Hornblower has spent a year at his home in Smallbridge, recuperating from the typhus he suffered after the siege of Riga. Now he is recalled to active service and is given the thankless task of recapturing a British brig, the Flame, whose crew has mutinied owing to the cruelty of their commander, whom they are threatening to hang unless they are given a full pardon. They may also seek refuge in France if they are attacked. Hornblower sympathises with the mutineers but knows that there is no possibility of a pardon. He recaptures the ship by a ruse and secures a French prize into the bargain.
A French official taken prisoner in the action tells Hornblower that the port of Le Havre is ready to secede from Napoleon in favour of the Bourbon cause. Hornblower enters the port and takes it over. His immediate task is now to secure it against attack. He manages to destroy a force that has been sent to recapture it but his friend Bush is killed while leading the action.
Once the situation is stabilised a French duke, a nephew of the King and his probable future heir, arrives, He is accompanied by Hornblower's wife, Lady Barbara. Hornblower's happiness at his wife's arrival is not unalloyed. He finds it difficult to combine the demands of active service with domesticity and relations become somewhat strained.
Napoleon falls from power and is exiled to Elba. There is general rejoicing among his enemies and Hornblower receives the news that he is to be elevated to the peerage; he is now Lord Hornblower of Smallbridge. This marks the zenith of his professional career, but on a personal level things are about to fall apart.
Hornblower's brother-in-law, Marquis Wellesley, is to go to Vienna to represent Britain at the Congress which is to decide the future of Europe, and he wants Barbara to act as his hostess. She is thrilled but Hornblower refuses bad-temperedly to accompany her and says he will return to Smallbridge. Before leaving Paris he and Barbara encounter the Comte de Graçy and his daughter-in-law, Marie, with whom Hornblower had an affair while escaping from France four years earlier. Hornblower is profoundly disturbed by seeing her again.
Alone in Smallbridge, Hornblower is unable to settle, and on an impulse decides to go to Graçy. He is made welcome there and renews his passionate affair with Marie. He is so ecstatically happy that he thinks of staying in France permanently. But then comes news that Napoleon has escaped from Elba and is once more Emperor. Hornblower, the Count, and Marie decide to lead a guerilla band against Napoleon's forces in the area. But things go wrong; Hornblower and the Count are captured and Marie is killed, leaving Hornblower broken-hearted.
Both Hornblower and the Count are sentenced to death, but on the morning they are due to be shot news comes of Napoleon's defeat at Waterloo, so they are reprieved. Hornblower is left wondering what will become of his life and his marriage.
The story thus ends on an unexpectedly tragic note. But it accords with Hornblower's character, reserved on the surface but deeply emotional within, afflicted with constant self-doubt that never allows him to feel contented for more than a brief time.