This is book 7 of the "Hornblower" series.

This note contains spoilers for the book.

Hornblower is a prisoner in Rosas, after fighting four Spanish ships and basically destroying them all (with just a little help from a followup British squadron).

He and Bush are taken by French police to be tried for war crimes (in a show trial) and executed.

Along the way, they manage to escape in a snow storm, take a boat down the Loire for a ways, get dumped at a dam, and find perhaps the only person in France who both would and could harbor them—a count who returned to revolutionary France because his sons wanted to, lost his sons in the army, and is also mayor of his town.