Post Captain is the second historical novel in the Aubrey–Maturin series by Patrick O'Brian, first published in 1972. It features the characters of Captain Jack Aubrey and naval surgeon Stephen Maturin, and is set in the early 19th century during the Napoleonic Wars.

During the brief Peace of Amiens, Aubrey and Maturin live in a country house in England, where they meet women with whom they fall in love. The mores of courtship restrict both men as to making marriage proposals. Then their lives are turned upside down when Aubrey loses his money due to decisions of the prize court and a dishonest prize-agent. To avoid seizure for debt, they proceed through France to Maturin's property in Spain. When the war begins afresh, Aubrey has a command aboard HMS Polychrest, gaining fewer prizes yet succeeding in his military goals. He is eventually promoted and is given temporary command of the frigate HMS Lively while its captain is ashore. The emotions of his love life interfere with his ways at sea, showing him sharply different in his decisiveness at sea compared to his clumsiness on land.

The novel was received well at its initial publishing, but received more and better notice after its re-issue in 1990. That much of the story is set on land drew some to consider it O'Brian's homage to Jane Austen, one of his favorite authors. Author Mary Renault gave the novel high praise, while Alison Sulentic commended it for the two different ways that Maturin and Aubrey "come to know wisdom" as a result of falling in love.

Plot summary With the Peace of Amiens, Jack Aubrey returns to England and rents a house with Stephen Maturin, with shipmates running the household, and the friends spending their time fox hunting. They meet the Williams family, who live on a neighboring estate. Aubrey courts Sophia Williams, the eldest of three daughters, while Maturin pursues Diana Villiers, Sophia's cousin. Jack wants to marry Sophia, but they delay making a firm engagement. His fortune disappears when his prize-agent absconds with his funds and the prize court finds that two merchant ships he had captured were owned by neutral nations. The court demands he repay the value of the ships (rather than gain the prize money he expected), a sum beyond his means. Mrs Williams takes her daughters away to Bath on this news. Jack also dallies with Diana, straining his friendship with Stephen and showing himself indecisive on land, in contrast to his decisive ways at sea. Aubrey and Maturin flee England to avoid Aubrey being taken for debt.

In Toulon to visit Christy Pallière, the French captain who had captured Aubrey's first command Sophie before the peace, they learn that a renewal of the war is imminent. French authorities round up all English subjects. Jack and Stephen escape over the Pyrenees to Stephen's property in Catalonia, with Stephen disguised as an itinerant bear trainer and Jack as the bear, Flora. They reach Gibraltar, where Jack and Stephen take passage aboard a merchant ship of the British East India Company, the Lord Nelson. The ship is captured by the French privateer Bellone, but a British squadron overtakes them and rescues Aubrey, Maturin, and the other passengers.

In England, Jack is offered a letter of marque by Mr Canning, a wealthy Jewish merchant. At the same gathering at Queeney's, Mrs Williams and her daughter Cecilia are among the guests. Unaware Jack would be there, Sophia had stayed home with her sister Frances. Mrs Williams learns of Stephen's castle in Spain and his training as a physician, raising his status in her eyes. An inadequate thief approaches Jack as he walks outdoors; this Mr Scriven proves to be a useful friend, knowing the law of debt and where Jack can be safe from bailiffs. Aubrey and Maturin move to The Grapes, safe in the Liberty of the Savoy.

Offered command of HMS Polychrest, Jack turns Canning down. Polychrest is an oddly designed ship that was purpose-built as an experimental weapon, though the project is now abandoned and the Admiralty has tried to refit her for ordinary service. She is structurally weak and sails poorly, and the first lieutenant, Parker, is liberal with his punishments of the crew. Jack asks that Tom Pullings be promoted to lieutenant. Jack is given a free hand by Admiral Harte, who stands to benefit personally from any prizes taken. To Harte's disappointment, Jack captures no prizes. When he drives the French privateer Bellone aground outside a Spanish port, the merchants reward him. Harte then assigns Jack to escort convoys in the English Channel. Jack gains a reputation for lingering in port as he carries on a furtive affair with Diana. Meanwhile, Stephen is sent on an intelligence-gathering mission in Spain. On his return, Stephen is advised by Jack's friend Heneage Dundas to warn Jack about his reputation with the Admiralty. When Stephen does so, Jack gets angry, their competition over Diana is acknowledged, and the two agree to fight a duel. Jack calls on Diana, but finds her with Canning, ending Jack's interest in her.

Jack is ordered to raid the French port of Chaulieu to sink the French troopships and gunboats gathered there and to destroy the Fanciulla. The crew plans to mutiny because of their harsh treatment under Parker, but Stephen overhears their plans and warns Jack. Jack rues his angry words with Stephen, and then quashes the mutiny by putting the instigators and some loyal crew in a ship's boat and quickly beginning the attack. During the engagement, Polychrest runs aground between two enemy batteries. Jack leads three of the ship's boats to board and capture Fanciulla. The successful party then attempts to refloat Polychrest, which founders after leaving Chaulieu, and the crew transfer to Fanciulla. After the battle, Stephen and Jack, who has been seriously wounded in the battle, resume their friendship.

Aubrey returns to England in Fanciulla and is promoted to post-captain. With debt still hanging over him, he asks for any available command. He is assigned as acting captain for , whose Captain Hamond has taken leave to sit in Parliament. Returning from Spain, Maturin tells the head of naval intelligence, Sir Joseph Blaine, that the Spanish will formally enter the war in alliance with France as soon as four ships full of bullion from Montevideo arrive safely in Cádiz. At Stephen's urging, Sophia asks Jack to transport her and her sister to the Downs. Though Jack is too poor to propose a marriage settlement satisfactory to Mrs Williams, Jack and Sophia come to an agreement not to marry anyone else. Stephen is close friends with Sophia but does not take her advice to propose to Diana. While attending the opera, he sees that Diana is being kept by Canning, and his pain is deep.

Maturin takes no pay for his intelligence work but does ask a favour: that Lively be included in the squadron sent to intercept the Spanish treasure fleet. The Admiralty agrees and asks Stephen to negotiate the fleet's surrender. Stephen is given a temporary naval rank; because of this and his connection to the Admiralty, Jack realizes that Stephen has been involved in intelligence work for Britain. Jack understands that there is a side of his friend that he did not know. The Spanish convoy refuses to surrender, and battle breaks out. One Spanish frigate (the Mercedes) explodes, and the other three (Fama, Clara, and Medea) surrender. Clara strikes her colours to Lively, pleasing Jack, but Fama, carrying the bulk of the Spanish treasure, attempts to escape to Cádiz; Lively pursues and captures her. Jack invites two of the Spanish captains to dinner, along with Dr Maturin, and they all toast Sophia.

Principal characters

Jack Aubrey: Commander of Polychrest and later appointed captain of HMS Lively. Stephen Maturin: Ship's surgeon, natural philosopher, friend to Jack and intelligence officer; son of Irish and Catalan parents, knows both countries well. Sophia Williams: Eldest daughter of three, fair-haired beauty of strong character, who lives in the neighborhood where Jack Aubrey takes a house during the brief peace. Aubrey falls in love with her but is indecisive with her in contrast to his decisive nature at sea. Mrs Williams: Mother of Sophia, Cecilia, and Frances Williams, widowed. She is protective of her daughters while working to find a suitable husband for each. An unpleasant personality. Diana Villiers: Same age cousin of Sophia Williams, both orphaned and widowed in India, now living with her Williams cousins. Both Stephen Maturin and Jack Aubrey are attracted to her dark-haired beauty and abrupt ways. Stephen surprises himself that he can fall in love again. Earl Saint Vincent: First Lord of the Admiralty when Aubrey first requests a ship, after prize court decisions go against him. He does not have enough ships at hand. General Aubrey: Father of Jack Aubrey. He recently married his dairymaid, and they have a son about the time Jack Aubrey learns of his debt due to the adverse prize court decision. The general lives at Woolcombe, is a member of parliament, and speaks out too often, and against the Royal Navy, thus against his son's career advancement. Sir Joseph Blaine: Head of naval intelligence, Stephen Maturin's contact at the Admiralty, and an entomologist. Christy Pallière: Captain of the French ship that took Sophie, with cousins in Bath whom Jack Aubrey visited when also visiting Sophia. He is affable and sociable, and a good ship captain. Dr Ramis: French ship's surgeon and contact for Maturin in Toulon. Dumanoir de Plessy: Captain of the French privateer Bellone, wounded while taking the Lord Nelson. Maturin treats his wounds, yielding kinder treatment to the passengers now prisoners on his prize. Azéma: Second captain from Bellone, captain aboard the prize Lord Nelson merchant ship, who struck his colors on meeting four Royal Navy ships of the line. Mr Adam Scriven: An unemployed literary man and translator who instructs Aubrey and Maturin in the laws of debt and the safe haven of the Duchy of Lancaster, the Liberty of the Savoy in the midst of London. Mrs Broad: Runs The Grapes, the inn in the Savoy, where Aubrey is safe from bailiffs wanting to take him for debt. He and Maturin set up there. Lord Keith: Admiral in the Royal Navy, married to Queeney. Queeney: Wife of Lord Keith, and in earlier days, neighbour and tutor to young Jack Aubrey, who hosts a gathering where Aubrey and Maturin meet Canning, as well as Diana Villiers, Mrs Williams and Cecilia. Heneage Dundas: Close friend of Jack Aubrey who is already made post captain, into HMS Franchise, when he meets Aubrey and Maturin at Queeeney's "rout" (large evening party). Mr Richard Canning: Wealthy trader with a fleet of merchant ships. He offers Aubrey a position as captain on his new privateering ship built like the Bellone. A married man, he pursues Diana Villiers, who goes into keeping with him. Lord Melville: First Lord of the Admiralty when war resumes. His political support is the opposition to those who supported his predecessor, Lord Saint Vincent. Admiral Harte: He has a squadron in the Downs, to which the Polychrest is attached. He has a strong grudge against Aubrey. Mr Parker: First Lieutenant in Polychrest, he is cruel and excessive in handing out punishments, which nearly results in a mutiny on the Polychrest. He had sailed with Prince William (Duke of Clarence), who pressed Lord Melville about his friend's progress. Ironically, Aubrey's good word about him helps him gain his step at age 56 to master and commander on the Fanciulla, bought into service after capture. Mr Tom Pullings: Young Second Lieutenant in Polychrest, promoted from master's mate, after his stint with the British East India Company during the peace, on the ship Lord Nelson, which Aubrey and Maturin board at Gibraltar.