The ninth of twenty volumes in O'Brian's classic nautical series, "Treason's Harbour" ties together some of the plot threads from the previous book, "Ionian Mission", but leaves some of its own questions unanswered. Ship's surgeon Stephen Maturin is back in the intelligence game in Malta, battling French agents and feeding them false information until he and Captain Aubrey are sent to the Red Sea. Their mission is disappointing, though, because of a mole in the British command whose identity is unknown but who also apparently foils the mission to Algeria that ends this book.

Two-thirds of the way into "Treason's Harbour", readers will finally hear about the capitulation of the town of Marga, which Aubrey's crew was besieging in "Ionian Mission". Similarly, in this book we never learn the fate of the Fieldings, he a naval officer escaped from a French prison to return to his wife and she with reputation ruined by Maturin's espionage machinations. This is a letdown, because we've spent so much time learning about them and watching Maturin considering their fates. We never learn the denouement of the Zambra mission but are left hanging after Aubrey has confronted three French vessels, this too is disappointing because the naval actions in this book are subdued, far less gripping than in other installments.

"Treason's Harbour" is a good book, skipping along with O'Brian's intelligent prose and complete ease with maritime matters, but is not wholly self-contained. The diving bell is fun, but this reader is still wishing to know about the Fieldings. The plot bobs and eddies but never quite runs out before the pages do.