Post captain is the 2nd book in the Aubrey-Maturin series
The book begins in 1802 with the conclusion of the French Revolutionary Wars and the start of the Peace of Amiens. Commander Jack Aubrey returns to England to take up the life of a country squire. He meets the Williams family, and their cousin Diana Villiers. Aubrey courts Sophia Williams (the eldest daughter), but is also attracted to Diana, with whom he commences an affair.
Aubrey plans to marry Sophia Williams, but his fortune soon is gone between an order to repay a captured merchant, the Cacafuego, being undersold, and the rest of his money being embezzled by his prize-agent. Aubrey has to flee the country to avoid going to debtors' prison and he plans to live with Maturin in Spain.
Before he gets to Stephen's estate in Spain, France declares war and begins rounding up English subjects. Having been tipped off by the French captain who had captured him in "Master and Commander", Stephen is able to smuggle Jack out of the country dressed in a bear costume. Finally making it through Gibraltar, Jack and Stephen take passage aboard an East India Company ship. Unfortunately, the ship is captured by the privateer Bellone. Discovered to be a Royal Navy officer, Jack is going to be sold to the French, but a British squadron overtakes them and rescues Jack and Stephen.
Returning to England, he is restored to active service by the outbreak of war in 1803. While waiting for a command, Jack is offered a letter of marque by a Mr. Canning. Jack ultimately turns Canning down and later is given command of HMS Polychrest, an odd ship that was designed to launch a secret weapon. The ship is a poor sailing ship and his endeavours are made harder by a sadistic lieutenant. Placed under the command of Admiral Harte, with whose wife Jack had an affair in the previous book, Jack is given a free hand in the hope that his lucky streak of capturing prizes will continue. This mostly doesn't happen as Jack's most notable action is driving the privateer Bellone aground outside a Spanish port, while allowing several merchants to escape. Disappointing Admiral Harte, Jack is assigned to escort convoys up and down the channel. During this time, he gets a reputation for lingering in port as he carries on an affair with Diana.
Meanwhile, Stephen is sent on a secret mission to gather intelligence in Spain. Upon returning, Stephen is advised by Heneage Dundas, a close friend of Jack's, to warn him about visiting Diana. When Stephen does, Jack is angry and accuses Stephen of lying to him as to where he had been during his absence. Soon they challenge each other to a duel. While in port, Jack calls on Diana, but finds her with Canning. As the main reason for the duel, jealousy over Diana, has vanished, he is willing to drop the duel but does not tell Stephen. Prior to the date of the duel, Jack is ordered to raid the French port of Chaulieu to sink the gathered French troopships and gun boats and to destroy the corvette Fanciulla. On the way, the crew plans to mutiny because of the treatment they receive from Lt. Parker. Stephen overhears their plans and goes to Jack, the first time they have spoken since the challenge. Forewarned, Jack separates the instigators of the mutiny from the rest of the crew.
During the engagement in Chaulieu, the Polychrest runs aground. Jack leads three boats to board the Fanciulla. After a short battle, the Polychrests capture the ship and pull off the Polychrest marooned on a sand bar. However, after hours of pounding by the shore batteries, the Polychrest founders and sinks soon after leaving Chaulieu. After the battle, the duel is forgotten by both Stephen and Jack, who returns in the Fanciulla to England and is posted captain. Jack is offered a ship that is currently being built but will not be ready to sail for six months. Afraid of being captured by his creditors, he declines and asks for a temporary command. He is assigned to HMS Lively whose Captain, Hammond, has just left to take up his seat in Parliament.
Stephen is again sent to Spain to gather more intelligence. This time, he returns with news that the Spanish will declare war as soon as four ships full of gold from Montevideo are safely in port. While Stephen is gone, Sophia, at Stephen's urging, asks Jack to transport her and Cecila to the Downs. While on board, Sophia and Jack come to an agreement not to marry anyone else; Jack is too poor to propose anything more. While waiting on the Admiralty's orders, Stephen attends an opera, and observes Diana sitting in a box with Canning. This stuns Stephen so badly that he is still sitting in his box long after the opera ends, eventually having to be told to leave by the ushers.
Fearing that a change in parliamentary leadership will leave Jack penniless, Stephen asks that the Lively be included in the squadron sent to intercept the Spanish. The Admiralty grants this request, assigns Stephen the title of captain pro tem (so he will receive a generous share of the prize money) and tasks him to be a negotiator. Because of Stephen's temporary rank and the now obvious connection to the Admiralty, Jack realizes that Stephen has been a spy for Britain.
The Spanish convoy refuses to surrender and a quick battle breaks out. One Spanish galleon explodes and the other three surrender. The victorious captains' share of the prize money will be enough to pay off Jack's debts.