When war is duly declared Hornblower leads a dangerous sortie on land to destroy a semaphore singalling tower and blow up a magazine. Other actions include a duel with a much stronger French ship, the Loire and Hornblower's snuffing out of the fuse on a shell that lands on his ship. But perhaps the most dangerous threat he has to face is a series of prolonged storms that threaten shipwreck and make it impossible for fresh drinking water to be supplied. He finally makes port with only two days' half-rations left.
When Hornblower arrives in England he finds that Maria has given birth to his first-born, a boy. Soon he has to go to sea again, this time as part of a force that is sent to intercept a Spanish fleet bringing treasure from the New World. Cornwallis has arranged this in the hope that there will be rich prize money available that Hornblower can share in. The Hotspur is caught up in action against a French frigate and so misses out on the capture of the treasure, but as it turns out none of the British ships involved are entitled to any prize money owing to a technicality.
So at the end of the book Hornblower is no richer than he was at the beginning. but he receives something that he values even more. Cornwallis, who is about to retire, has the right to make promotions before he leaves, and he has made Hornblower a full Captain.