Captain Aubrey leads a small squadron in the Mediterranean, with orders to sabotage new French ships a-building along the Dalmatian coast, and to intercept a treasure on the way from North Africa to the French. There are some momentous personal experiences (which I obviously don't tell you, not wanting to spoil the story).
If you're new to the Aubrey/Maturin stories, DO NOT START WITH THIS BOOK: go right to the beginning, Master and Commander, and begin there. You have the treat of a lifetime ahead of you, you lucky thing! But this is the 19th in a series, and it pays to know the personalities, the backstory, the authorial foibles. Yes, I agree with other reviewers, this is not the strongest book in the series, and there are some really sad moments; but there is nothing, nothing, to persuade me to give fewer than five stars (and I do not give five stars easily!). For the admirer, there is much to enjoy in this story; for the comparer, there are none to touch O'Brian and his heroes.