Treason's Harbour

Treason's Harbour is the ninth novel of the Aubrey-Maturin series by the English writer Patrick O'Brian, first published in 1983. It is set during the Napoleonic Wars, and for much of its length it keeps the series' two principals in one place, ashore at a Mediterranean station.

The Surprise is laid up for repairs after a successful cruise, and the officers fill the weeks with the music and the society of the port. Among the residents is Laura Fielding, whose husband is a prisoner of war in France, and whose position makes her useful to the French intelligence agents operating on the island. Maturin works out what is happening to her and decides that helping her is worth the risk.

A change of command at the station then puts Aubrey back to sea, on errands that take him across the Mediterranean and towards the Red Sea, some of them in borrowed ships. Two of those errands turn out to be traps set for him rather than the work he was told they were, and the book follows him through them, as Maturin works on what he has begun to suspect about the island's intelligence network.