At the start of this story Hornblower has been married to Barbara for several months and is the squire of the village of Smallbridge, a position that he doesn't enjoy. He is therefore delighted when he is summoned to the Admiralty and offered the appointment of Commodore to take a squadron of ships to the Baltic. His new command comprises the seventy-four-gun Nonsuch, two sloops, two bomb ketches, and a cutter.
Hornblower is briefed on his mission by his brother-in-law Arthur Wellesley, who is Foreign Secretary. He tells Hornblower that the political situation in the Baltic is delicate. The Danes are allied to Napoleon. Sweden is ruled by Bernadotte, who three years previously had been a French general, although he is officially neutral at present. But the major uncertainty is the position of Russia. The Czar wants to avoid war but it is possible that Napoleon will attack him anyway. In that case Russia will almost certainly be defeated and Hornblower is to evacuate the Czar if he is ousted; meanwhile he should offer any help he can.
Once in the Baltic Hornblower destroys a French privateer and carries out a successful raid on some French vessels in harbour, but these are relatively minor affairs. The situation changes after Napoleon invades Russia; Britain and Russia are now allies. Hornblower finds himself involved in the defence of Riga, which is besieged by the French under the command of General Macdonald. The town looks likely to fall, but news comes to say that Napoleon is retreating from Moscow. The Russians are literally dancing with joy, but the siege is not yet lifted and the British cannot leave Riga until it is. The French are only defeated after a desperate last struggle, in which Hornblower leads a cavalry charge against an invading French column,
At the end of the book Hornblower is left totally exhausted by fatigue and cold. He is also suffering from typhus, as we learn at the beginning of the sequel. He sinks into a delirium in which he hallucinates himself back in England with Barbara and Richard.
Readers who lack a clear mental picture of the geography of the Baltic may find the story a little difficult to follow at times; the publishers would have done well to include a sketch map of the area.