The Mauritius Command is the fourth book of the Aubrey-Maturin series of Novels.
Jack Aubrey is on half-pay at home with his wife Sophie, his twin baby girls, and shrewish mother-in-law Mrs Williams. Dispite being happy with his wife and kids, he misses the sea and command. His spirits are lifted when Stephen Maturin comes to call, and brings great news. Aubrey is ordered to take command of the 38-gun frigate HMS Boadicea, and sail to the Cape of Good Hope, and take command of a squadren as Commodore. As he readys to set sail, the wife of one of his squadren's Captains, Lady Clonfert, seeks passage with Aubrey to enable her to join her husband. Aubrey is not keen on this, and leaves without her one morning.
The long trip was mostly uneventful, and spent mostly on trying to bring his crew up to decent standards, which was not fully successful. They meet with the French ship Hébé which is escorting a captured merchant ship. After a brief chase the French are overcome and the ships occupied. Hébé turns out to be HMS Hyaena captured some time before by the French. He sends the prizes to Gibraltar under the command of the Boadicea's aged First Lieutenant Akers. Aubrey uses this device to be rid of the officer and send home letters, one of which attempts to excuse his leaving early without Lady Clonfert.
On arrival, Aubrey meets Admiral Bertie and also has to contend with the disparate characters of his captains. One of these is Lord Clonfert, a minor member of the Irish aristocracy who has political influence, and who served with Jack Aubrey whilst out in the West Indies. They were involved in an action together and he had some reservations at the time about Clonfert's courage. Another is Captain Corbett who is a harsh disciplinarian and drives his men almost to the point of mutiny. Barett Bonden, usually Aubrey's coxswain, and Preserved Killick request permission to join Aubrey once more, particularly as Bonden was given fifty lashes for an unpolished firing piece on his gun.
During his campaign Aubrey temporarily switches his pennant to the elderly 64-gun ship of the line HMS Raisonnable, but returns to the more seaworthy HMS Boadicea with the onset of the tropical typhoon season. La Réunion is captured almost bloodlessly after a landing by British East India Company troops under the cooperative Colonel Keating, their path already softened up by Maturin's propaganda and political machinations. Mauritius proves a tougher nut to crack. As HMS Néréide is detached to chase the Iphigenia to Port South East on Mauritius, Maturin suffers a serious fall and spends much time in the company of Lord Clonfert and Mr. McAdam, Clonfert's learned but drunken surgeon. The first demonstrates himself to be a largely ineffective person, craving the fawning attentions of his officers and crew, whilst McAdam, a less convivial conversationalist, is made fun of by the young officers particularly when "in his cups."
However, events worsen on their arrival. Keen to capitalise on the capture of the Île de la Passe fort, the small group of ships, under the command of the unadventurous but solid Captain Pym, land men and troops to consolidate the land campaign. Whilst so disposed, the French appear with four ships Bellone, Minerve, Victor & Ceylon. They boldly sail past the fort into the port, the British are caught unprepared but decide to sail in to attack. They struggle to navigate the unfamiliar channel into the harbour and, with two British ships running aground, the French are able to bring all their guns to bear on the ships that eventually reach the harbour. The end result is the Néréide is taken (Clonfert is severely wounded in the neck and head by a wooden splinter), Sirius and Magiciénne are burnt to prevent their capture, and Iphigenia and the fort Île de la Passe are abandoned to be retaken by the French. Only a messenger vessel, with Maturin aboard, gets back the La Réunion to inform the commodore of the "ill tidings".
Aubrey immediately rushes to see if Iphigenia and Île de la Passe can be saved but the British are chased off after finding both are clearly in the French hands. After eventually making contact with the Emma transport and the Windham, which itself appears to be unseaworthy, Aubrey believes his fortunes have changed when HMS Africaine - now commanded by Captain Corbett - re-joins them. Sailing in chase of the French during the night Africaine, clashes with the Astrée and the renamed Iphigenia (once again the French Iphigenie). But the encounter goes badly and Corbett is killed during the fight, probably, as the ship's surgeon informs Maturin later, by his own oppressed men. The French capture the ship, but leave it dismasted when the Boadicea and Aubrey bear down on them and, much to Aubrey's joy, refuse an engagement. Joined by the Otter and Staunch, the flotilla eventually reaches harbour and Africaine's refit is the Commodore's top priority.
Before repairs are complete the Pearl races towards harbour, meeting HMS Boadicea with the news that Bombay is nearby, being pounded by both the French Vénus and Victor. Outrunning the Staunch and Otter, Jack engages the pair who have captured Bombay and makes use of extra volunteer crew from the refitting HMS Africaine to board both Bombay, recapturing her, and the Venus. During the encounter the French Commodore, Hamelin, is killed by grapeshot in his heart. Now with news that Bellone and Minerve are almost certainly "heaved down", and Iphigenia and Néréide are likely to be of little use even if refitted, Aubrey believes the tide has turned in his favour. En-route from St. Denis to take Mauritius from the French, the squadron encounters a large British force under the command of Admiral Bertie, who proceeds to steal Aubrey and Keating's thunder by taking command of the whole invasion force and claiming the honours. However, news of the birth of his son causes Aubrey to remain ebullient even when everyone expects his mood to be downcast.
The final invasion, based almost entirely on Aubrey and Keating's original plan, is almost without bloodshed. The French capitulate after being given honourable terms, and Maturin finds that Clonfert has committed suicide by removing the bandages from his wounds whilst captured, unable to face up to the jubilance of his rival, Jack Aubrey, in victory. A ceremonial dinner is given back in Cape Town and Admiral Bertie, under the impression that Aubrey has influential political connections, gives Aubrey the honour of taking the dispatches aboard the Boadicea and sailing for England in compensation for "stealing" his victory.