The film begins in January 1793 as a 17-year-old Hornblower joins a ship of the line, the Justinian. Hornblower is introduced to his shipmates, including Jack Simpson, a bully who rules the midshipmen's quarters. Hornblower does not distinguish himself when he becomes seasick while the ship is at anchor in calm waters. Hornblower considers suicide under Simpson's persecution and finally finds opportunity to challenge him to a duel, even though Simpson is an experienced and deadly duellist. An older midshipman, Clayton, feels guilty that he himself has not stood against Simpson. He knocks Hornblower unconscious, takes his place in the duel and is killed. Simpson, though wounded, survives. Hornblower is transferred to the frigate Indefatigable, under the command of Captain Pellew. Midshipmen Kennedy, Hether, and Cleveland go with him, as well as Simpson's old division which, due to Pellew's ire, becomes Hornblower's division. Hornblower's conduct in battle eventually wins the respect and loyalty of these men. Simpson joins the Indefatigable when the Justinian is sunk by a rogue French ship called the Papillon. Pellew sends a detachment on a cutting out expedition in the Gironde to take the Papillon. During the battle, Simpson shoots Hornblower, and sets Kennedy, incapacitated by a seizure beforehand and left in a boat, adrift. Meanwhile the Indy is attacked by three French ships. The Papillon, now in British hands, comes under fire from shore batteries, and the two officers senior to Hornblower, Lieutenants Eccleston and Chadd, are killed. Shortly before the most senior officer dies, he tells Hornblower to take over. Simpson attempts to take command as the senior midshipman, but Hornblower tells Mr. Bowles, the ship's master, that if Simpson resists, "you have my permission to shoot him". Simpson is then taken below whilst the Papillon sails to the Indys rescue. Hornblower orders the Papillons French colours not be lowered. Through this ruse of war, he launches a surprise attack on the unsuspecting French ships. After the British victory, Hornblower accuses Simpson of attempted murder, Simpson challenges him to a second duel, which Hornblower accepts. In this duel, Simpson treacherously shoots Hornblower early, before the combatants have been told to fire. Simpson claims that it was a misfire. Hornblower is not badly injured. Simpson is then told to stand his ground and await Hornblower's shot. Faced with this prospect, Simpson begs for his life, revealing himself to be a coward. Hornblower fires into the air, stating that Simpson is "not worth the powder". Infuriated at this insult, Simpson attempts to stab Hornblower in the back, but is shot dead by Pellew just before he reaches him. Back on the Indefatigable, Pellew tells Hornblower precisely how impressed he has been by his actions and that Hornblower has a great career ahead of him, if he continues as he has begun.
This episode does not follow the plot of the corresponding chapter in C. S. Forester's Mr. Midshipman Hornblower. In the original story, the mathematically minded Hornblower secures himself an "even chance" against a more skilled adversary. As the offended party, he has the choice of weapons. He asks that only one of the duelling pistols be loaded, the combatants having to toss for it and then stand a yard apart and fire at each other at point blank range. As it turns out, the captain has secretly given orders that neither weapon be loaded, the resulting lack of a shot blamed on a, and Hornblower comes out of the duel unscathed. Suspecting the truth, Hornblower proposes to challenge the captain to a duel, only to be told that such a challenge would be unlawful. The episode also incorporates material from several other stories in Mr. Midshipman Hornblower. Among these are "Hornblower and the Cargo of Rice", for Hornblower's ill-fated command of the Marie Galante, and the chapter "Hornblower and the Man Who Felt Queer", for the section concerning the Indefatigable's expedition against the Papillon. The episode also diverges from the plot of this chapter in several respects, mostly related to the addition of Simpson and Kennedy to the event.
