Sherlock Holmes and Dr. John Watson are in Holmes’ Baker Street apartment, examining a walking stick left at the apartment the evening before by an unknown visitor. The stick was a gift, as shown by a plaque on it that says it’s from “friends of the C.C.H.” to James Mortimer. Using this information, Holmes asks Watson to determine what he can about their mysterious guest. Watson deduces from the stick’s worn appearance that it belonged to an older man, successful and well-esteemed—an old country doctor, who makes his rounds on foot. Holmes encourages Watson’s speculation but eventually reveals much of what Watson’s said to be false.
Rather than an elderly old country doctor, Holmes supposes Mortimer to be a young surgeon in London, well-liked as Watson supposed, but who recently abandoned his position for the country due to a lack of ambition and not because of age. Mortimer, he says, also owns a dog—one larger than a spaniel but smaller than a mastiff. Watson confirms most of Holmes’ deductions using a directory of medical professionals. The remaining information comes shortly thereafter, when Mortimer returns in search of both Holmes and his walking stick. Holmes has erred only in that Mortimer left the London hospital because he got married (although, Mortimer does lack ambition).
Mortimer has come to present Holmes with a unique case, which he’s sure will interest the famous detective. The case begins with a document from 1742 that tells the legend of Hugo Baskerville and how he was mauled to death by a hound—allegedly from Hell—that came to be known locally as the hound of the Baskervilles. Hugo had kidnapped a local townswoman with the intent to rape and otherwise brutalize her at his home in Baskerville Hall. However, before Hugo had the chance to enact his awful plans, the woman escaped. The legend states that Hugo agreed to sell his soul to the devil if only the devil helped Hugo to catch the woman before she made it back to town, and the devil made good on his side of the bargain. Hugo ruthlessly killed the woman, but he was quickly made to pay his due, as he was found shortly thereafter being devoured by a large, hellish hound. This hound, the legend states, will stalk and kill any Baskerville caught traversing the moors outside of Baskerville Hall at night for at least four generations: a veritable Baskerville curse.
Mortimer has brought this strange legend to Holmes because Sir Charles Baskerville has recently died under what appear to be equally strange circumstances: his heart gave out while running from something. While the townspeople seem happy with the official explanation that Baskerville died from over-exertion on his evening walk, Mortimer has discovered the footprints of a large dog near the body. He doesn’t want to rile up the moor’s inhabitants by suggesting that these footprints might belong to the hound of the Baskervilles, however, so he keeps the information to himself, sharing it only with Holmes.
Mortimer has not come to Holmes for help in solving the mystery of Sir Charles’ death, though. Rather, he wants Holmes’ advice on how to handle the coming of Sir Henry Baskerville, Sir Charles’ only living heir, who, having inherited Baskerville Hall, is moving to his new home from America. Mortimer worries that Sir Henry might quickly come to the same end as Sir Charles, should he choose to live at Baskerville Hall. Though he’s a man of science, Mortimer is tempted to believe that the curse might be true. Holmes agrees that the case is an interesting one and agrees to think about it some more. He asks Mortimer to bring Sir Henry to the Baker Street apartment as soon as possible, without telling him anything about the Baskerville curse or the hound.
Upon their subsequent meeting, Sir Henry reveals that he has received an odd letter at his hotel, and that one of his new boots has been stolen. The letter, he shows Holmes, was constructed almost entirely of words cut from a newspaper. It warns Henry to stay away from the moor if he wants to live. Holmes is intrigued and quickly realizes that the words were cut from the previous day’s Times newspaper. Where the author of the letter has been forced to print certain words, such as “moor,” he or she has gone to great lengths to obscure the handwriting. Even here, however, Holmes is able to deduce that the author likely used a pen from a hotel. How the author knew where to send the letter, though, remains a mystery to all. No one knew at which hotel Sir Henry would be staying when he arrived from America. Concerned, the group repeats the legend of the hound of the Baskervilles to Sir Henry, and Holmes advises the man not to go to Baskerville Hall. Henry, however, flatly refuses this advice, saying that no devil in Hell could keep him from his ancestral home.
When Sir Henry and Mortimer leave together, Holmes and Watson shadow them. Holmes is convinced that someone must be following Sir Henry, since there’s no other way that his hotel address could have been known. Sure enough, they quickly discover a bearded man following Sir Henry from a horse-drawn cab. The man realizes he’s been seen, however, and makes a quick escape—though not before Holmes gets the number of his cab. Holmes uses this information to get the name and address of the cab driver. At the same time, he also hires a boy to scour the wastepaper of local hotels in search of the newspaper that was used to create the odd letter sent to Sir Henry Baskerville. He also sends a telegram to Mr. Barrymore, the bearded housekeeper of Baskerville Hall, to see whether or not he is at home. If he’s not, Holmes surmises, it’s possible that he was the one following Sir Henry.
Later, at lunch, Sir Henry reveals that he’s missing another boot: an old one this time. The group also discusses Sir Charles’ will. He was, it seems, a great philanthropist. Mr. and Mrs. Barrymore, the housekeepers of Baskerville Hall, inherited some 500 pounds each from Sir Charles, and Mortimer inherited 1,000 pounds. In total, Mortimer tells the group, Sir Henry’s inheritance will be some 740,000 pounds in liquid assets that, when combined with the value of the estate, total nearly one million pounds (over 100 million pounds in today’s currency). Obviously, Holmes declares, it is this money that is providing the motivation for the crime. Holmes reluctantly agrees that Sir Henry must go to Baskerville Hall if they’re to get to the bottom of things, but he implores Watson to go as well. Watson agrees, happy that Holmes has shown such confidence in him. Accompanying Sir Henry to his room, the group is surprised to discover the new boot that Sir Henry was missing in a place where they’d already searched for it. Additionally, Holmes receives two telegrams: the first informing him that Mr. Barrymore was indeed at home, the second reporting the failure of the hired boy to find the cut-up copy of the Times. This leaves Holmes with but one last lead: the cab driver. Holmes meets with the driver and offers him money in exchange for any information he might have about the bearded man. The cab driver only knows that his passenger claimed to be the famous private detective, Sherlock Holmes. With this information, Holmes suddenly has no leads at all in the case.
It isn’t long before Watson, Mortimer, and Sir Henry make their way to Baskerville Hall. During the trip, they encounter heavily armed soldiers. The soldiers are searching for Selden, a murderer who has recently escaped from a nearby prison. Upon arriving at the Hall, Mortimer departs, and Sir Henry, accompanied by Watson, is greeted by Mr. and Mrs. Barrymore. Mr. Barrymore soon suggests that the couple may not want to continue on as housekeepers for long. They worry that the younger Sir Henry will want to live more grandly than did Sir Charles—at any rate, they haven’t felt the same about the Hall since Sir Charles, who they considered a friend as well as an employer, died. Exploring the house after dinner, Watson and Sir Henry are impressed by the long line of family portraits. Later, in bed, Watson hears the distinct noise of a woman crying. The next morning, Watson and Sir Henry ask Mr. Barrymore about this crying. Barrymore claims it wasn’t his wife, but Watson notices shortly that Mrs. Barrymore’s eyes look as though she’d been weeping. Watson begins to suspect Mr. Barrymore of being a brute towards his wife, which leads to further suspicions. Watson decides to go to the postmaster’s office and enquire whether the telegraph that Holmes sent to Barrymore was delivered directly to Barrymore’s hands, and no one else’s, as directed. Sure enough, he finds that the message was left with Mrs. Barrymore: so it is once again possible that Mr. Barrymore was the bearded man who had been following Sir Henry.
While returning from the post office, Watson encounters Jack Stapleton, who knows a surprising amount about the case: that Sir Charles had a weak heart, that there was a legend about the hound of the Baskervilles, and that Sherlock Holmes has taken an interest in the case. Stapleton is particularly keen to discover Holmes’ thoughts on the situation from Watson and offers his assistance in whatever way possible. Together, the two walk back to Stapleton’s house so that Watson can meet Beryl Stapleton, who Jack introduces as his sister. Along the way, Stapleton points out a particularly nasty part of the moor that is mostly swamp. Together, he and Watson witness a wild pony tragically drowning in the marsh and hear an inexplicable howling, as if from a great hound. Stapleton claims that he’s the only person on the moor who can safely navigate the swampy area—it’s a skill he’s had to learn as a naturalist, since the best butterflies and rarest plants can be found there. Adjacent to this area, Stapleton also points out a series of Neolithic huts that he claims still bear evidence of being lived in by early humans. As Beryl Stapleton walks up to meet the two men, Jack is distracted by a rare butterfly and runs off. Beryl hurriedly approaches Watson and urges him to leave the moors as quickly as possible. When Jack returns, however, she instantly abandons this line of talk and, when Jack reveals that his guest is Dr. John Watson, she blushes and informs Watson that she thought he was Sir Henry Baskerville. Later, when Watson is walking back to Baskerville Hall, Beryl sneaks out of her house to catch up to him, asking that Watson forget her earlier warning. Her fear, she says, was just a flight of fancy that she can’t really explain.
In a report written to Holmes about the ensuing days, Watson reveals that Sir Henry has met and taken a liking to Beryl Stapleton, though Jack Stapleton seems strongly disapproving of this, even flying into an outrage when he catches the two together alone. Jack agrees, however, to accustom himself to the idea of Sir Henry dating his sister, if Sir Henry will only give him a few months to do so. Most importantly, however, Watson reports that he and Sir Henry have discovered Mr. Barrymore using a candle to signal to someone on the moor late at night. When confronted, Barrymore refused to explain himself, even when Sir Henry threatens to fire him. The mystery is only solved when Mrs. Barrymore reveals the secret in order to save their careers. Mr. Barrymore is signaling to Selden, the escaped convict. The Barrymores have been providing Selden with food and clothing (including some clothes which Sir Henry had donated to Mr. Barrymore), because Selden is Mrs. Barrymore’s brother, and she feels responsible for him. Both Watson and Sir Henry see the impossible situation that the Barrymores have been put in and forgive them for their subterfuge. Still, Watson and Sir Henry feel that they must try to apprehend the criminal and immediately take to the moors to attempt it. Though they are athletic, young men, Selden quickly outpaces them and gets away. In their pursuit, however, Watson sees another man in the distance, watching them. They are unable to catch this mysterious man, either, and Watson is only able to confirm that the man did not appear to be any of the neighbors that he has met. He now believes that this man is the same one who followed Holmes in London and that Mr. Barrymore was only a red herring.
The next morning, Mr. Barrymore thanks Sir Henry for allowing him to continue working at Baskerville Hall (although Barrymore doesn’t like that Sir Henry tried to hunt Selden down). In exchange for this graciousness, Barrymore reveals something he’d kept secret from everyone out of respect for Sir Charles Baskerville: namely, that Sir Charles was scheduled to meet a woman at the exact hour and location where he was killed. Mr. Barrymore doesn’t know much beyond this, just that a letter arrived for Sir Charles that same day requesting the meeting. It was written in a female hand and signed “L.L.” After some inquiry, Watson decides that L.L. must be Laura Lyons, a typist in a nearby town to whom Sir Charles had sometimes given charity in the form of money. Watson goes to Lyons, and learns that she had planned on asking Sir Charles for the funds necessary to obtain a divorce from her husband, but she found an alternative source for these funds and thus never went to the meeting. She never cancelled the plans, however, and will tell Watson nothing about her other benefactor.
On his way back to Baskerville Hall from Lyon’s house, Watson witnesses a boy delivering food into the moors and, following the boy’s path, finds himself amongst the Neolithic huts he’d observed earlier. Here, Watson comes face-to-face with none other than his friend Holmes, who has been living in the huts the entire time, in order to get an outsider’s view of the situation. At first Watson is upset by Holmes’ deception, but Holmes praises Watson’s work so far. This cures all of Watson’s ill-will. In comparing reports, Holmes fills Watson in on some facets of the case that Watson still has not unearthed: most importantly, that Jack and Beryl Stapleton are not brother and sister but rather husband and wife. Holmes deduces that Jack has pretended to be a single man in order to ensnare Laura Lyons into his plan, using her to lure Sir Charles out into the moor where he was killed. Holmes assumes that Jack plans to use Sir Henry’s infatuation with Beryl Stapleton similarly. Both now agree that Stapleton is the man responsible for Sir Charles’ death and the threat to Sir Henry’s life. Holmes and Watson are still working out all of the details when they hear a baying and snarling hound nearby. Fearing that Sir Henry is in danger, they rush towards the sound only to find the body of a dead man. As the body is dressed in Sir Henry’s clothes, they fear the worst. However, on closer inspection, they realize that the body is none other than Selden, who is wearing the clothes that Sir Henry donated to Mr. Barrymore. Selden seems to have fallen to his death while trying to escape the hound—which Holmes and Watson now believe to belong to Jack Stapleton. Helping to confirm their suspicions, Stapleton arrives at the scene of the death only moments later, sure that Sir Henry has been killed and seeming slightly disappointed when he has not.
Back at Baskerville Hall, Holmes and Watson reveal nothing of their suspicions of Stapleton to Sir Henry. Instead, they bid him to simply follow whatever directions they give him, without question. While they talk, Holmes takes great interest in the family portraits hanging on the wall, noting that Sir Henry has features not unlike those of Hugo Baskerville, the would-be rapist whose death started the legend of the hound of the Baskervilles. However, when Holmes uses his hands to cover up a portion of Hugo’s features, a stronger resemblance is seen. Hugo Baskerville looks exactly like Jack Stapleton. From this, Holmes gleans Stapleton’s motive: he must be a hitherto unknown member of the Baskerville family and heir to its fortune should Sir Henry die. Holmes remarks that he now has only to be able to prove what he knows in order to arrest Jack Stapleton. He tells Sir Henry that he and Watson will be returning to London immediately to better work on the case, and advises Sir Henry to have dinner with Stapleton that night. Holmes warns Sir Henry to only take the straight path through the moor between Baskerville Hall and Stapleton’s house. Watson is astounded by this advice and Sir Henry terrified by it, but both agree.
Attempting to find the proof they need, Holmes and Watson again visit Laura Lyons. When they reveal to her that Jack Stapleton is a married man, she tells the two everything. Stapleton had convinced Lyons that he wanted to marry her, if only she could pay for a divorce from her estranged husband. Such a marriage would have been a life-changing event for Lyons, who barely managed a living through typing work and was a societal outcast as an estranged woman. Stapleton suggested that Sir Charles would happily give her the money and demanded that she arrange a meeting with him. When the time arrived, however, Stapleton told her that it would be inappropriate to ask Sir Charles, and that Stapleton himself would find the money for the divorce. Clearly, this allowed Stapleton to know precisely where Sir Charles would be at the given hour, allowing Stapleton to know when and where to release his hound. Lyons assures Holmes and Watson that she never knew Stapleton intended harm to Sir Charles, a man she cared deeply about because of all the help he’d given her.
Holmes reveals to Watson that he has no intention of returning to London. He only wanted to make Sir Henry and Jack Stapleton believe that he would be gone, so that Stapleton would feel emboldened enough to attempt to murder Sir Henry. On their way back to the moors, Holmes and Watson meet with a detective from Scotland Yard, who Holmes has enlisted to help them in capturing Stapleton. Together, the three men set up watch on the road outside of Stapleton’s house. They’re sure that Stapleton will release the hound the moment that Sir Henry leaves from his dinner there. As they wait, however, a thick fog rolls in, forcing them to retreat further and further from the house in order to avoid being wrapped up in it. Eventually, they place themselves out of sight on the roadside and wait for Sir Henry to pass them by. Moments after he does, the men hear the sound of a dog running their way. Soon, a massive beast gallops past them. It appears in the fog to be breathing fire and to have glowing red eyes. The men are taken aback by this at first, and it passes them unharmed on its way to Sir Henry. Holmes, however, quickly springs to his feet and running at an astonishing rate catches up to the beast just as it pounces on Sir Henry. Holmes shoots the dog dead, saving Sir Henry.
Rushing to Stapleton’s house to apprehend him, the Holmes and Watson find only Beryl Stapleton, who Jack has mercilessly tied up in a bedroom because she refused to continue to help him in his crimes. Jack has fled into the swampy part of the moor. The men go after him, but soon come to believe that he has been pulled down into the marshes, a victim both of his own haste and the dense fog. They find the secret area where Stapleton kept the hound as well as Sir Henry’s old boot, which Stapleton used to provide his dog with Sir Henry’s scent. Later, inspecting the dead hound, they come to realize that its glowing eyes and fire-breathing mouth were merely a phosphorous paint that Stapleton had applied. The tale ends with Holmes, months later, lengthily wrapping up the remaining loose ends for Watson. Many of the details in Holmes’ description are redundant, but others—such as his identification of Stapleton as the mysterious bearded man who trailed them in London—are new information.