This is book 9 of the "Aubrey-Maturin" series.

This note contains spoilers for the book.

They're in Malta waiting for repair of the Surprise. A virtuous but social and musical woman, Mrs. Fielding, becomes friends with both of them. Aubrey rescues her dog from a well, and the dog becomes so attached to Aubrey that everybody assumes they are having an affair. The French, however, are blackmailing her with threats to her husband (who is a prisoner), and set her on Stephen. Stephen doesn't get to take her to bed either; but he figures out what is going on, and starts using her to send false information to the French (perhaps his favorite hobby).

They're sent to see if they can capture a Turkish galley hauling French silver over in the Red Sea. They make a trip across the Sinai and embark, and find the galley, and figure out that it's a trap. The sink the galley, and confirm that the French have been there for more than a month.

Maturin has one of Halley's diving bells, and he and the Rev. Martin go down to examine the galley. They rescue some of the cargo, and determine that it's lead, with a note saying they're fools.

Jack learns that the Surprise is going to be sold out of the service when they return to England, and that he's not to have the Blackwater after all (it's been given to somebody else). And he realized this means that the Admiralty official who has mistreated him will also be his enemy in future.

Turns out Mrs. Fielding's husband has escaped, and he shows up and hears rumors of his wife's infidelity, and is morose and angry. He asks Jack for a meeting in Malta (a duel).

Finally they and the Pollux (with Harte aboard, but explicitly excluded from the negotiations) head to Zambra bay to deal with more French. But, again, the French have things sewn up, and a squadron comes up. The Pollux is destroyed, and the Surprise just barely manages to escape (through Jack's superior local knowledge).

These Mediterranean books have some remarkable characters in them. The Turks, from pashas and deys to common seamen, include some remarkable characters (including an obidashi who speaks perfect lower-deck English, because his mother was English). And there's Haribediean, who everybody likes; but he will insist on swimming in the sea, and the sharks get him. And he turns out to have stolen Jack's diamond-encrusted chelengk.

In the end of this one, Stephen knows two key French agents, and doesn't know who to trust. He thinks Wray is a possibility (and we the readers know better). But Wray is away.