Having achieved fame and financial security, Captain Sir Horatio Hornblower has married Lady Barbara Leighton (née Wellesley) and is preparing to settle down to unaccustomed life as the squire of Smallbridge in Kent.

He still yearns to serve at sea and accepts with alacrity when the Admiralty appoints him a commodore, puts him in command of a squadron and sends him on a diplomatic and military mission to the Baltic.

Hornblower is shown dealing with the problems of squadron command, and using naval mortars (carried on special ships known as bomb vessels) to destroy a French privateer.

Hornblower narrowly averts a major diplomatic incident when his secretary and interpreter (a Finnish refugee assigned to him by the Admiralty) attempts to assassinate the Tsar at a court function.

In some editions of the novel the story ends here with the hallucinating Hornblower imagining himself being greeted in Hampton Court by Lady Barbara and his infant son.

British bomb vessels had not been ketch-rigged since the 1780s; the mortars were crewed not by the bomb-vessels' officers and men but by specialist detachments of the Royal Marine Artillery; and many other points.

In The Commodore (but not in the real Napoleonic period), as in the Second World War, the Royal Navy offered substantial help to Russia: at the siege of Riga, and by guarding the Arctic convoys.