Sailing into Halifax, the victorious HMS Shannon contends with her losses in officers and crew, with particular concern for Captain Broke, who lies unconscious from head wounds. The American Captain Lawrence dies en route from the battle, and is buried at Halifax. Once in port, as prisoners of war are taken ashore, and the British Navy deserters identified among them, the Shannons and her passengers, Captain Jack Aubrey, Dr Stephen Maturin, and Mrs Diana Villiers feel the full joy of the first naval victory in this war with America. Maturin communicates with Major Beck, an army counterpart in intelligence work. At the victory ball, Aubrey is pursued by Amanda Smith, known to Diana for her deceiving ways. Aubrey tires of her after a night, yet she persists. Aubrey receives his first letters from his wife Sophia since the Leopard was left in the Dutch East Indies, so long ago. Broke cannot write the report of his victory, so others do it, to speed the official news to England.
Captain Dalgleish on the mail packet Diligence carries the copy of the official report, and Aubrey, Maturin and Mrs Villiers as passengers. The American privateer Liberty chases Diligence on its northern route home. Diana is certain that the privateers are hired by Johnson, who wants his vengeance. The Liberty sails into ice and sinks, her crew taken aboard by her follower, and Diligence reaches the Channel in 17 days. News of the victory is well-received, while Aubrey is eager to get home. He sees his children, grown so much from when he last saw them, and his wife Sophia. Maturin visits Ireland for his uncle. He gives Johnson’s private papers to Sir Joseph Blaine, asks him for Diana's release, and gets Skinner as a lawyer for Aubrey to deal with the projector. Maturin goes to Paris, to present his scientific work at the Institut , taking Diana with him. He finds her a place to stay with Adhemar de la Mothe and an accoucheur as Diana is pregnant by Johnson. At the Institut presentation, Diana wears her diamonds. She dearly loves the diamonds, including the largest, called the Blue Peter. After Maturin speaks, he learns of Ponsich’s death near Pomerania. Maturin leaves immediately to take up this mission.
Letters from Miss Smith discomfit Aubrey. Maturin advises Aubrey that Miss Smith is lying. Maturin wants Aubrey as his captain to reach the heavily fortified Grimsholm Island in the Baltic and Aubrey is ready for sea. They are joined by Jagiello, a Lithuanian from the Swedish army to translate. Blaine tells Maturin that Ramon d’Ullastret is the leader in the fortress; Maturin knows him well. Aubrey is offered the sloop HMS Ariel , leaving on the next tide, with no time to stop home for his usual sea chest. Mr Pellworm, a Baltic pilot, is on board when Aubrey arrives at Ariel . Ariel passes Elsinore where shore guns fire but miss the ship. At Carlscrona, they meet with Admiral Saumarez to devise their plan. Aubrey takes the Minnie , the privateer carrying French officers to Grimsholm, uses it to carry wine, tobacco and Maturin. When Maturin begins speaking in Catalan, he is accepted and no lives are lost as the British take the fortified island. Admiral Saumarez welcomes Colonel d’Ullastret and is pleased with their success. The Colonel boards the Ariel , while the Catalan garrison travels in troop transports to Spain with Aeolus as escort, again navigating the narrow channels past Denmark. Before Ushant, a crew member drops the only chronometer, so they sail not knowing their exact location. In heavy rains, Ariel meets one French ship, the Méduse , and one British. Aubrey tells the troop transports to part, as Ariel will aid HMS Jason against the fast-sailing Méduse .
Ariel crashes into a rock, trying for the safety of Douarnenez Bay on the French mainland. They are taken as French prisoners of war. Colonel d’Ullastret is given a Marine’s uniform and a false name, and he promptly escapes. Abruptly, Aubrey, Maturin and Jagiello are taken to Paris with Monsieur Duhamel and lodged in the Temple, being torn down around them. Aubrey works on a way out of the prison, with help from Jagiello. Maturin is questioned by competing French intelligence groups; from one he learns that Diana miscarried. Duhamel makes an offer from parties expecting the emperor’s defeat. From English newspapers, Aubrey learns that HMS Ajax took Méduse , making his efforts worthwhile, and that Miss Smith is married. In another session, newly arrived Johnson identifies Maturin as the killer of two French agents, after an interrogator says someone has paid "half Golconda" for his release. Maturin agrees to go with Duhamel, who takes them out of the Temple, picks up Diana, and remarks how Aubrey’s escape shaft will be the explanation for their disappearance. They board the packet ship HMS Oedipus , under William Babbington. Diana has given up the Blue Peter to a French minister, a diamond from the Golconda mines, to save Stephen. They marry on the ship, with Aubrey giving her away, and Babbington officiating.