This is book 7 of the "Aubrey-Maturin" series.

This note contains spoilers for the book.

Once again Aubrey and Maturin are sent home (from Halifax, the British naval base in Canada, where the Shannon carried them) in a despatch boat. This one is strangely dogged by a pair of American privateers, which ignore richer prey to chase them specifically. It becomes apparent that Mr. Johnson is aboard, and is pursuing them seriously—because of Stephen's intelligence activities, Diana's leaving, or perhaps the fact that she took her diamonds with her.

Stephen then goes to France to address the Institute, and is approached by many people trying to get him to compromise himself. He expected this, and does not, of course. Diana stays on as a guest of a friend of Stephen's.

They then get sent on a Baltic mission, to persuade Catalan troops serving Napolean to desert him. Since they're commanded by Stephens godfather, and are all men he knows, this proves quite doable. This mission introduces Lieutenant Jagiello, a Lithuanien in Swedish service; a beautiful young man, very popular with the ladies, to the extent that "the Gentleman's Relish" comes aboard and climbs into his bed.

On the way back from that, they end up nearly sinking in French waters, and docking at a French port and surrendering. They're taken to an old prison, in the process of being torn down, and various groups question Stephen, ending with Mr. Johnson arriving from America and identifying him (Stephen took most of Johnsons private papers when he left).

One reason they're so sure Stephen is an agent, it turns out, is that Diana, who knows nothing about his being an agent, has offered her diamond for his safety (something no sane agent would do). This has convinced some of the military people that he must be very important, even if they don't know how yet. This nearly does Stephen in.

Just at this point, Royalist elements who are starting to see hope for their cause take Stephen and Jack and Jagiello, and even Diana, back to England. They even promise her her diamnod back.

It's amazing how much of what I remember of the series is contained in this early part—under half.