'The Duchess and the Devil' is the third installment of the Hornblower series and takes place- naturally enough- after the events of 'The Fire Ships'. As the story begins, Hornblower is leading a raid against a contingent of the French, who are sneaking supplies past the British blockade.
It is at this point we are introduced to a new character: Hunter, a midshipman under Hornblower's command. He isn't terribly fond of Hornblower, as Horatio's strategy differs significantly from his own- which is essentially to shoot, stab, or punch everyone. He says resentfully that he thought they were there to fight the French. Horatio replies that no, they are there to defeat them. Hornblower's plan works, and they take the French ship and supplies with no casualties among their own men, and very few among the Frenchmen. This increases the admiration which Hornblower's men have for him, except for Hunter, who thinks winning by outsmarting or tricking your enemy is cheating.
Back aboard the Indefatigable, Captain Pellew derives some amusement by teasing his overly-serious acting lieutenant, then tells Horatio that, in addition to the prize money he will receive for capturing the French vessel- La Reve- he will be given the honour of captaining her back to England. He is also given the dubious privilege of attending a dinner hosted by the governor of Gibraltar. Here Horatio makes the acquaintance of the Duchess of Warfedale, who confounds his assumptions about the comportment of women of the upper class by talking and acting in a manner more suited to a tavern wench. He also has it sprung on him that he is going to be transporting the Duchess back to England aboard La Reve.
As Horatio prepares La Reve to leave for England, Capt. Pellew gives him a packet of dispatches to be delivered to the Admiralty in London. He warns Hornblower that they cannot fall into enemy hands; if he is boarded or captured, the papers must be destroyed. Then the Duchess arrives, and Pellew wishes him luck and scuttles off, looking more discomposed than he ever has under cannon fire. The Duchess continues her teasing of "Mr Haitch" who is still young enough to blush at her slightly risque banter. Unfortunately, the Duchess is soon the least of his problems as they run into some Spanish ships and are captured. Horatio is about to toss the dispatches overboard, but the Duchess convinces him to give them to her for safekeeping, saying that she won't be suspected of having important documents. He is soon given cause to have second thoughts about this course of action, as the duchess begins chatting up their captors in fluent Spanish, and is whisked away to stay at the warden, Don Masserado's casa in comfort while the rest of them are tossed into a Spanish prison.
Poor Horatio... in jail, and forced to share a cell with Hunter, who naturally blames Hornblower for the entire situation. After all, if he had allowed them to fight to the death like Hunter wanted, they wouldn't be in prison, would they. But there is another occupant to their cell; Horatio is shocked to find that Archie Kennedy is also there. It seems that after Simpson set his boat adrift, he was picked up by the Spanish and has been imprisoned here ever since. Curiously, Archie is not happy to see his former shipmate. He is in bad shape physically as well as mentally; having attempted escape numerous times, he was punished by being put in an oubliette- a hole in the ground not large enough to stand up or lie down in- for a month. Now lacking the strength to get out of bed, the sight of Horatio brings back haunting memories of Simpson, causing mental anguish and a return of his seizures.
Hunter, of course, has no time for this weakness. He wants to escape, and Kennedy will only slow them down. Hornblower tells him that they will escape eventually, but will be taking Archie with them. The Duchess hasn't been idle during this time: she has convinced Don Masserado to allow Horatio to go walking with her outside the prison once a day if he will give his word that he won't try to escape. Hornblower promises that he won't try anything during the walks, but informs Don Masserado that at all other times, he considers it his duty to attempt escape.
These walks give Horatio the opportunity to ascertain that the Duchess still has the dispatches safe, but are poor optics for his men who are stuck in prison 24/7. This wouldn't be a big deal, except Hunter spends his time suggesting to them that Hornblower doesn't want to escape- that he would be happy to sit out the war, swanking around with the Duchess. Matthews and Styles keep it together, but the others, including Oldroyd, are wavering.
When not sowing seeds of rebellion, Hunter is busily chowing down on Kennedy's rations. Having lost the will to live, Archie has stopped eating. Distracted by other matters, Horatio doesn't notice this. He no doubt would have seen Archie's untouched food about, except Hunter is eating it, and doesn't bother to mention that little fact to Horatio. Hornblower finds out when Kennedy lapses into unconsciousness and cannot be roused. Distraught and blaming himself, Hornblower carries Archie to the gates of the prison, asking for help. Don Masserado allows Kennedy to receive medical attention, and Horatio, with some help from the Duchess, cares for him.
At first, Archie resists eating, not wishing to recover. Horatio insists, saying that Kennedy must get well and return with them to the Indy. Archie indicates he doesn't want to go back and listen to tales of how he was rescued by Hornblower. Horatio tells him that Archie would do the same for him if he was in that state, and Archie replies with sad bitterness that Horatio never would be. Regardless, Hornblower manages to convince him to eat by telling him that he needs him if he's going to get the men out of here. As he recovers, Archie catches sight of the Duchess and... recognizes her. He informs Horatio that she's not an aristocrat- she's Kitty Cobham, a Drury Lane actress from London. Horatio is left pondering the chilling thought that he has handed secret government documents over to a woman who has lied about everything, including her name. While he is distracted, Hunter plots his own escape with the men who have thrown in with him.
When confronted, Kitty explains that, when war broke out, she was stranded in Europe and impersonated the Duchess of Warfedale in order to get back to England. And, since her portrayal of the Duchess was dead-on, she doesn't see what difference it makes. Their argument is cut short by a visiting French official, who has also recognized Kitty. She buys his silence by agreeing to spend the night with him. Horatio spends a bad night, torn between being scandalized by her choice, and worried that it won't stop the Frenchman from spilling the beans... if Kitty is imprisoned, no doubt the dispatches will be discovered. The next day when they meet for their walk, Horatio attempts to reproach her for her behaviour, but Kitty slaps him down, pointing out she sacrificed her pride and dignity for their continued safety. She also assures him that the dispatches are still safe, and she'll keep them that way. Soon after this, Horatio learns that she obtained transport on a Spanish vessel and is gone.
Back in the prison, Archie is up and about, and Horatio's mind is turning to escape plans. Knowing that Hunter is plotting behind his back, he tries to reason with him and the other men. He points out that they don't know yet how many guards there are, or what their schedule is. Also, he asks Hunter what he plans to do, if by some miracle they do break out: loose in Spanish territory, dressed as Englishmen, and unable to speak the language to ask directions or get a boat. He also mentions the fact that Archie, whom Hunter wants to leave behind, speaks Spanish. Hunter dismisses all these concerns as nonsense- once they get out, they'll make it up as they go along. Horatio orders them to give up the ridiculous notion, but of course they don't.
Hunter puts his so-called plan into action, and naturally it ends in abject failure... one of their men is killed, and Hunter is shot in the leg. Don Masserado demands to know who was responsible, and Hornblower states that he was. Masserado doesn't believe Hornblower would be that stupid, and tells him to give up whoever was actually to blame, warning him that, as his friend Kennedy could tell him, punishment will be cruel. Horatio insists that he be held responsible, even with Archie quietly urging him to tell Masserado the truth. Unconvinced but resigned, Masserado orders Horatio put in the oubliette.
Above ground, Matthews and Styles give a remorseful Oldroyd a hard time for listening to an idiot like Hunter. Speaking of whom, he is a broken man; wracked by guilt and pain, he is refusing to try to get well. Ironically, it is Archie who cares for him, bandaging his wound and telling him he must eat, that Horatio will need him. Eventually, Don Masserado relents and Hornblower is freed, though he's in bad shape, unable to stand. Time passes as both Horatio and Hunter recuperate, and Horatio's parole is reinstated by Don Masserado, allowing him a walk a day. It is while on one of these walks, during a storm, that Hornblower comes upon Don Masserado on the shore, watching a sea battle. Borrowing his spyglass, Horatio sees that it is the Indefatigable, chasing a Spanish vessel. In an effort to escape, the Spanish ship comes too close to a rocky reef, running aground and starting to sink. Horatio asks if anyone is going to try to rescue the crew, but Masserado says that the storm is too bad... no one will go out in it, as it would be suicide. Hornblower asks for permission for he and his men to take out a boat and attempt a rescue. Don Masserado is suspicious that he is looking for an avenue of escape, but Horatio gives his word that he and his men will return if they survive. Masserado agrees, and they manage to make it out to the ship, rescuing the survivors, which Horatio is shocked to find include Kitty. Hunter bravely jumps into the sea and saves one of the Spaniards, but is washed away himself. The storm is so bad that they can't return to shore, so they try to ride it out asea. While huddling together to keep warm, Kitty explains her presence. It seems that the ship has had a hard time dodging the British, so was unable to drop her off at a neutral port. She assures Horatio, however, that she still has the dispatches safe in her clothes. Fortunately, they are spotted and picked up by the Indy, still lurking in the area.
Pellew is obviously delighted, though he huffs a bit when the "Duchess" reveals that she has the dispatches. She praises Hornblower highly, however, and Pellew tells them that he is no longer an acting lieutenant. Due to his courageous actions during the Fire Ship incident, his rank has been raised to Lieutenant. Horatio is pleased, but informs Captain Pellew that he and his men must return to their imprisonment, as he gave Don Masserado his word. Pellew thinks this is carrying honour a bit too far, and says that, while Hornblower can return, the men must decide for themselves whether or not to accompany him.
Pellew puts the question to the men, and Archie says that Horatio's word stands good for him. Pellew asks the others if they feel the same,and it is Oldroyd who speaks up stoutly, and says that they, too, will abide by Hornblower's promise. The Indefatigable takes them in as closely as possible to shore, and just before disembarking, Horatio asks Kitty what she plans to do- she can't keep pretending to be the Duchess of Warfedale. Kitty says that she'll drop the act once she gets back to London: she has friends there who will overlook the rather unorthodox manner of her return.
Horatio and his men return to prison, but not for good. Some time after their re-incarceration, Don Masserado comes to see Hornblower and tells him that, due to their valiant rescue of the Spanish sailors, he and his men are being set free. Don Masserado wonders audibly if their Majesties realize they are releasing someone who will prove to be a great thorn in their side. Horatio replies that he will endeavor not to disappoint.