Jack Aubrey, now a civilian, prepares the Surprise to sail as a letter of marque. The loss of his place on the Navy list is the hardest blow. He is stoic and figures he will get used to it, but appears harsh instead of his normal happy outlook. His reputation brings him a full crew, and he takes the men on liking. He runs the Surprise on Royal Navy lines, including regular pay to the men, in addition to any prizes they might take. He is supported by his crew of old Surprises, privateers and smugglers, the latter groups recruited in Shelmerston, on the western coast of England. It is let out that a group of his friends purchased the ship at the auction, as Stephen Maturin, who is the sole owner, wants to play his same role of surgeon and natural philosopher on the ship. The mission to South America will happen after Aubrey takes the new crew on a short cruise in the Atlantic.
The downfall of the traitors Wray and Ledward restores order in British intelligence circles, returning Sir Joseph Blaine to his position in the Admiralty. The traitors managed to flee England, so they still have a friend in the government. Duhamel, the French agent who gave them away, never did reach Canada, as he died in a fall boarding Eurydice . Blaine says it will be difficult to clear Aubrey of the charge and restore him to the Navy, even with solid evidence left behind by Wray showing how he profited in the stock market scheme and set Aubrey up. Maturin's servant Padeen becomes a secret laudanum addict after a painful burn during gunnery practice, where he learned its benefit, followed by an infected painful tooth that Maturin could not treat. Padeen dilutes the ship's supply with brandy. Maturin is thus unknowingly weaned off his own addiction.
During the short cruise, the Surprise captures an American privateer's consort, the Merlin . Learning where the American/French privateer Spartan seeks its next quarry– the valuable cargo of quicksilver aboard the Spanish barque Azul – Surprise sails to intercept. Azul is struck on rocks, with Spartan adjacent following a fierce battle between the two; Surprise meets them and boards first Azul and then Spartan . Aubrey tricks the Spartan's five prizes out of Horta harbour, making him and his crew wealthy, improving his reputation further, and earning him a gift of silver plate from the merchants who had been so harried by Spartan . Blaine tells Maturin of a mission to intercept the frigate Diane , a French ship ready to voyage to South America. Aubrey plans the attack at night in ship's boats, cooperating with the Navy, specifically William Babbington of HMS Tartarus , who has made post Captain, thus removing pressure on him to take credit for success. Surprise takes the Diane and all other vessels in the French port of Saint Martin-de-Rey the night before Diane planned to sail. Maturin imprisons the intelligence agent aboard, taking his papers.
In the short clash on the Diane , Maturin kills her captain. Aubrey took a bullet near his spine. The second success makes Aubrey a popular hero. He is offered the opportunity to request a free pardon, but angrily declines on the grounds that he is innocent. Aubrey's father, a fugitive since his part in the stock-jobbing affair, is found dead in a ditch. Aubrey organizes the funeral for him, which takes him to his boyhood home of Woolcombe, now his by inheritance. After the funeral, Edward Norton (a friend of Aubrey's grandfather) offers Aubrey a seat in Parliament, from the borough of Milport. This gain in position leads Lord Melville, First Lord of the Admiralty, to assure Aubrey of his restoration. Aubrey is a changed man.
Maturin travels to Sweden to speak to his wife Diana Villiers. Aubrey will meet him there for the return voyage. In Stockholm, Maturin purchases a bottle of full-strength laudanum and some coca leaves from a well-stocked apothecary. He meets Diana near her home in Stockholm. He learns Wray lied about not finding Diana in London to deliver the letter; she saw Wray, and no letter was given her. Maturin explains why he was seen with Laura Fielding. Villiers assures him she has not been unfaithful with Jagiello, who is soon to be married. She makes some money by ascending in a hot-air balloon before an audience, unsuccessful in her plan to breed horses. He gives her the Blue Peter, the diamond she gave up to save him, which pleases her greatly. He tells her of his sudden increase in wealth. Maturin takes two doses of laudanum and becomes disoriented.
He is seriously injured in a fall, breaking his leg. Diana nurses him and they are reconciled. Surprise returns from a stop in Riga to buy poldavy. Martin tells Maturin that he caught Padeen diluting the laudanum supply with brandy, and that Padeen is addicted and in irons. They carry Maturin out to the ship in style, accompanied by Colonel Jagiello's escort, and Diana embarks with him for home.