Sherlock Holmes is sitting at the breakfast table one morning when Watson arrives and peruses a walking stick left by an unknown visitor to the Baker Street apartment the night before. Holmes invites his doctor friend to make his best guess about the mysterious visitor based on both the engraving and the stick itself; when he does, Holmes commends him and Watson is pleased with the praise for his deductions. Unfortunately, his conclusions were wrong; the praise was for inspiring Holmes to draw more accurate conclusions. The walking stick appears to belong to a young doctor who once worked in a London hospital and now practices medicine in a rural setting and owns a dog. A quick look at the Medical Directory shows Holmes to be correct, even down to the dog (a spaniel)—which Holmes sees as he rises and looks out the window. Holmes invites Watson to stay and expresses his anticipation of an adventure about to begin.

The visitor is Dr. Mortimer. Once he is settled, he presents his rather unusual problem. In his pocket he carries a document given to him by Sir Charles Baskerville, a friend who died three months ago. The letter, written in 1742, relates the story of a family curse placed on Hugo Baskerville, “a most wild, profane, and godless man.”

Hugo Baskerville is lord of a manor and has fallen in love with a young peasant woman who lives near his estate. On a night when he knows her father and brothers will be away, Hugo and five or six men kidnap her and hide her in an upstairs room. The frightened girl hears the drunken revelry downstairs and escapes down the ivied wall and strikes out on foot over the moors toward her home, which is three leagues distant. When Hugo discovers the girl has left, he returns to his men and unleashes his fury. One of the men suggests they release the hounds on her, and Hugo immediately acts on the idea. He orders a groom to saddle his horse and release the dogs; after giving them the girl’s kerchief, the hounds race in “full cry” over the moors by moonlight.

The entire thing happens so quickly that the drunken men do not at first respond to this dramatic event, but soon they grab pistols and flagons of wine and mount in pursuit. They pass a shepherd on the moors and ask if the maiden had passed by, but he is stuck dumb with fear and does not respond at first. When he finally speaks, he tells them he saw the maiden being chased by the dogs, but he also saw something else—Hugo Baskerville riding maniacally with a “hound of hell” in silent pursuit. The men move on and soon see Hugo’s mare, frothing at the mouth and riderless, heading their way. The men are frightened but go on to find their friend. Soon they see the bravest dogs cowering in fear at the edge of a gorge. A few of the more intrepid (or sober) men move forward to see the fallen body of the maiden, who died of fright and exhaustion. Then they see their fallen friend as a huge black beast, larger than any hound they have ever seen, tears his throat out. The men turn shrieking from the sight and never recover from the shock. Since then, many Baskerville men have suffered violent and mysterious deaths. Because of this, the letter warns, no Baskerville should ever be caught on the moors at night.

At the end of the reading, Holmes dismisses the story as a fairy tale until the doctor produces a relatively current newspaper article that recounts the death of Sir Charles Baskerville, the most recent bearer of the family name. He was a widower known for reviving the family’s fortunes and demonstrating his munificence to the town. The Barrymores are a husband and wife serving as butler and housekeeper, and they detail the events of the evening of Sir Charles’s death. As always, Sir Charles walked the Yew Alley of Baskerville Hall to smoke a cigar before retiring. That night he did not return. Barrymore found his body at the end of the Alley. The murder remains unexplained, though there are several bits of evidence from which to work. First, the man’s footprints appeared to have changed partway down the Alley; second, the man’s face was contorted to such an extreme that Dr. Mortimer did not believe it was his friend when he saw him. The coroner’s verdict was death from natural causes. The next Baskerville heir, Sir Henry Baskerville, a nephew, has been notified.

Holmes is a now a bit more interested and asks for any particulars that may not have been detailed in the newspaper account. Mortimer says he did withhold several things from the public report, fearing no one would ever again inhabit Baskerville Hall if he told all he knows. There was an incident several weeks before the death in which both he and Hugo caught a glimpse of some huge, otherworldly creature on the grounds; it was on this night that Mortimer received the letter from his friend. Also, twenty yards from Hugo’s body on the night of the murder, the doctor observed the footprints of a giant hound, which is something he would not have looked for if he had not known the legend. Others have reported seeing “a huge creature, luminous, ghastly, and spectral” roaming the moor. The doctor is convinced there may be supernatural elements at work. He is seeking the great detective’s advice regarding the wisdom of bringing the last Baskerville heir to such a doomed fate. Holmes asks for twenty-four hours to ponder, and Mortimer agrees to bring Sir Henry to see him tomorrow. Holmes spends the day studying a map of Devonshire and assimilating all the known data; Watson does the same.

In the morning their guests arrive, and the young baronet explains that he would have come to see Holmes even if the doctor had not brought him—he, too, has a mystery to solve. He has received a clumsily crafted letter warning him to stay away from the moors if he values his life and his reason. It was delivered to his hotel room. He had only chosen that hotel at the last moment, so it is evident someone is following him. Also, one of his new boots was stolen or misplaced overnight. Dr. Mortimer tells Henry about the letter and the curse, and Henry insists that he will continue the journey to his ancestral home. The four men agree to meet again in several hours; when the visitors depart on foot, Holmes rushes Watson to gather his coat and hat in order to follow them and discover who is following the Baskerville heir. Holmes sees a man in a taxi with a large, bushy beard (probably fake) following the two men; when he endeavors to catch the man, Holmes is spotted and the man shouts for his taxi driver to make a getaway. Holmes berates himself for his ineptitude but does get the number of the taxi for further investigation.

At lunch, Sir Henry is angry that one of a second pair of boots has been stolen, although the hotel assures him they will be found. Holmes asks Mortimer if he has an acquaintance in Devonshire with a bushy black beard, and the doctor names Barrymore, Sir Charles’s butler. They establish, via telegram, that Barrymore is at the Hall. They also find Henry’s new boot in the very room in which they are dining. Furthermore, the amount of money left by Sir Charles after some small settlements and charitable donations is 750,000 pounds; however, the only one who inherits any significant money is Sir Henry, and the next in line for inheritance is a saintly man who refused even a small settlement from Sir Charles. The complexity of the mystery heightens for Holmes, and he asks Henry to allow Watson to accompany him when he goes to Baskerville Hall. Watson will keep Holmes apprised of events there while he finishes some important cases in London.

At Baker Street that night, the cab driver Holmes had followed comes to inquire into a complaint about his service. When Holmes questions the man, he gets very little useful information about the bearded passenger who left town on a train except his name—Sherlock Holmes. Holmes is amused by the audacity of the bearded man and expresses his misgivings about sending Watson on this trip. Watson assures his friend that he is armed and will take every precaution for his safety.

Before Watson leaves, Holmes asks him to send regular reports of the activities in Devonshire untainted by whatever theories Holmes is pondering. Sir Henry’s other boot is lost for good, and the three men board the train as planned. A waiting carriage takes them to Baskerville Hall. They leave behind them the more lush and fertile land and arrive on the moors, a dark, austere, and melancholy place. Dr. Mortimer spies an armed soldier on a hill and asks what has prompted such security. The driver tells him that an escaped convict, the Notting Hill murderer, has created fear in the area for the past three days.

Barrymore greets them when they arrive at the hall, and Dr. Mortimer goes straight home to his wife and waiting practice. Sir Henry purposes to install more lighting, for the hall is an intimidating sight in the darkness of the moors. Inside he shows his delight at the imposing structure, which has been his ancestral home for five hundred years. Barrymore is polite but tells the new Baskerville master that he and his wife will be leaving as soon as it is convenient. When questioned, the butler says they were enough staff for a reclusive elderly man but are not prepared for a young man and his future needs. When pressed further, Barrymore says they were too emotionally attached to their former master to continue on here without him. Neither excuse rings true. That night Watson has trouble sleeping; in the middle of the night, he hears a woman’s muffled sobbing but nothing more. The next morning things appear much less formidable to both men. When Barrymore is asked if heard a woman crying in the night, he denies hearing anything of the sort; however, his wife’s red, swollen eyes tell a different story. Sir Henry settles in front of a mountain of paperwork, and Watson heads to town.

Watson is dismayed to find that the telegram sent from London to establish Barrymore’s physical presence in Devonshire had failed, making the black-bearded butler the most likely murder suspect in Watson’s mind. Watson is hailed by a man with a tin container and a net, who introduces himself as Jack Stapleton of Merripit House. The naturalist addresses Watson by name and presumes the famous Sherlock Holmes is working on the Baskerville murder case. Stapleton has been in the area two years, knows (from Dr. Mortimer) that Sir Charles had a heart condition, and is adept at circumventing the deadly bogs of the Grimpen Mire on the moor. By paying careful attention to certain landmarks, he is able to navigate the dangers of the swampy spots and reach the habitats of the creatures he is studying. Suddenly they hear a low, haunting sound, a kind of strange moaning floating over the moor; Stapleton dismisses it as the sound of some nearly extinct bird, but Watson is not convinced. As Stapleton flits off to chase a butterfly, Watson is startled by a young woman who emphatically warns him to leave the moors. Stapleton returns and introduces his sister, Beryl, who is embarrassed to have mistaken Watson for Sir Henry. They go to Merripit House, and Watson is struck by how little resemblance there is between these siblings. On his way home, he is surprised to see Miss Stapleton waiting ahead of him. She explains that she ran ahead and must hurry back before she is missed but wanted to apologize for her earlier mistake. Watson presses her for the cause of her anxiety for Sir Henry, but she tells him nothing.

Watson writes his first letter to Holmes after nearly two weeks in Devonshire. He tells of the escaped convict still loose on the moors, the odd influence Stapleton appears to wield over his sister, and the site of Hugo’s murder, which Stapleton showed both Watson and Sir Henry. When they visit Merripit House, Sir Henry seems enamored of the young lady, and Watson observes that the feeling may be mutual. Since then they have all seen each other nearly every day, despite occasional looks of disapproval from Stapleton to his sister. The most interesting news concerns the Barrymores. Not only was Mrs. Barrymore crying in the middle of the night (a fact about which her husband lied), but Watson observes the butler walking through the house and into a room in the middle of the night, where he places a light at the window. He waits a short time, looking intently out the window, and then leaves the room.

Watson’s second report to Holmes is dated two days later. He discovers that the room into which Barrymore went has the nearest view of the moor, and he discusses the matter with his host at breakfast. Henry has heard him, too, and they plan to follow Barrymore and discover what he is doing the next time they hear him on one of his midnight wanderings.

After breakfast, the young man prepares to leave the house and Watson prepares to go with him. Sir Henry is meeting Miss Stapleton and leaves before Watson can catch up with him; however, Watson remembers Holmes’s insistence that the Baskerville heir never be left alone on the moors, so he follows. The scene is embarrassing for Watson to watch, and as soon as he sees Sir Henry again he confesses that he was a saw the entire episode. Sir Henry explains that he tried to kiss the young lady. She was reticent to return his affection, and then Stapleton interrupted then with a series of wild protestations regarding the wooing of his sister. In the end, the brother apologizes for his overreaction and asks that Beryl and Henry devote the next three months to cultivating a friendship with no talk of marriage. Sir Henry readily agrees to this plan—both to keep the peace and to continue seeing the young woman he loves.

That night the two men follow through with their plan to catch Barrymore in action, and they find him using the candle to signal someone on the moors. He refuses to reveal any details, saying it is not his secret to tell, and Sir Henry fires him. As they approach Eliza Barrymore, her husband tells her they must leave and she reveals the truth. Her brother, Seldon, is out on the moors; he is the escaped convict. He was petted and spoiled by the family all his life until he came to expect the rest of the world to provide for his every need. Seldon came to them one night, starving and pursued, and knew his sister would help him. When Henry arrived at the Hall, Seldon left and has been living on the moors; every other night, if there is a signal from the moor, Barrymore takes the outlaw some bread and meat. After he recants the firing, Sir Henry determines to go to the site of the signal and capture the renegade. Watson reminds him of Holmes’s warning about being alone on the moors at the hour of the curse, but Henry is determined and they head for the moor. Suddenly they hear the same awful baying sound Watson heard earlier, and Watson tells Sir Henry the villagers call it the hound of the Baskervilles. It is an unnerving and unsettling sound, but they proceed with their plan. They discover the felon hiding in an ancient hut; when he sees them he throws a rock at them and escapes. Neither man shoots. Watson sees one odd spectacle before they return to the manor: a tall, thin man standing on a hill, silhouetted against the moonlight.

Watson keeps a diary of his time in Devonshire and relies on it to tell the next part of the narrative. The morning after their adventure on the moors, Watson and Henry are both rather disgruntled. Watson is still pondering the silhouette on the moor and thinks perhaps he is the man who murdered Sir Charles. Watson determines to discover this man’s identity.

Sir Henry is still somewhat shaken by the haunting sound he heard on the moors last night, and he explains himself to Barrymore, who feels offended. The baronet feels justified in chasing down the prisoner on the moor; Barrymore assures his employer that his brother-in-law will be leaving for South America soon and swears the man will cause no trouble or do any harm before he goes.

Barrymore then shares a bit of information about Sir Charles’s death, something he did not learn until after the murder but has not yet shared with anyone else. He reveals that his former employer was at the gate that night to meet a woman. On the day of the murder, Sir Charles received a letter from Coombe Tracy, which was addressed in a woman’s hand and bore the initials L. L. Most of the missive had been burned in the fireplace grate, as requested by the sender, but Barrymore could read the request to meet her at the gate at ten o’clock. The letter has since crumbled. Barrymore explains he has not shared what he knows because his own family troubles have distracted him and also because he does not want to damage Sir Charles’s reputation. Watson immediately sends this information to Holmes, who is apparently quite busy because he writes very little to Watson in return.

One rainy night, Watson goes walking on the moor hoping to catch a glimpse of the mysterious stranger; instead, he meets Dr. Mortimer, who is looking for his missing cocker spaniel. Watson takes the opportunity to ask about a woman in the area with the initials L. L. and is told that a woman named Laura Lyons lives in Coombe Tracy. She is the daughter of Frankland (a cantankerous neighbor); she had some trouble and is trying to start a new life working at a typewriting business after her father disowned her.

Watson gleans one more bit of information as he visits with Barrymore after dinner. His brother-in-law has not been seen on the moors for the past three days, though the food Barrymore left has been eaten—either by him or the other man. Seldon told Barrymore about the other man who has been hiding on the moor: he is not a prisoner but a gentleman and has a young boy bring him food. Barrymore is distraught at the obvious truth that something is just not right; he wants Sir Henry to leave for his own safety and is equally anxious to leave the moors with his wife. Watson continues his pursuit of both the mystery man on the moor and the connection of Laura Lyons to Sir Charles.

Watson meets with Laura Lyons, who eventually admits she had asked Sir Charles to meet her the night he died. He was known for his benevolence, and she felt he would have helped her if they could have met; however, she did not go to Baskerville Hall that fateful night. She received help from another source and did not have a chance to get word to Sir Charles before he died. Watson leaves “baffled and disheartened.” He then contemplates the remains of the ancient huts that dot the moors and wonders about the stranger who is obviously hiding in them. On his way home, Watson is waylaid by Frankland, Baskerville’s neighbor. He shows Watson, through a telescope, a young boy taking a bundle of food to a hut on the moor. Watson leaves the neighbor and stealthily makes his way to the spot where the boy left his bundle. The hut is empty, but there are signs that someone is making his home here; the most shocking discovery is a note that says “Dr. Watson has gone to Coombe Tracy.” Watson’s logical conclusion is that he, not Sir Henry, is the target of this man’s interest. He determines not to leave until he meets this man, and soon he hears footsteps and a familiar voice. It is Sherlock Holmes, looking as meticulously groomed here as he ever did in London. After his initial joy at seeing his friend, Watson is offended that Holmes deceived and used him. Holmes assuages his wounded friend’s pride and assures him his work has been helpful in every way. Holmes explains that he hid for everyone’s greater safety.

When Watson tells of his visit to Laura Lyons, Holmes is reassured that his information is correct—there is “a close intimacy” between Stapleton and this woman. He also reveals that the Stapletons are husband and wife, not brother and sister. Watson is suitably shocked. Stapleton, they now know, is out to murder Sir Henry; they hope they can set a trap for the naturalist before he takes action.

Suddenly a horrific scream rings out across the moor, and the men run toward the sound, fearing the worst for the Baskerville heir. They come upon the body of a man who has fallen (or been chased) over a cliff: they find him face down, with his body contorted and his skull crushed. They berate themselves for being too late to help their friend and vow to avenge his foul murder. As they descend to the body, Holmes begins to laugh because the dead man is the escaped convict dressed in Sir Henry’s clothes, provided by Barrymore as a final assistance for the man’s escape to South America. Holmes wonders how the poor victim knew he was being chased by the hell hound, for sound alone would not have told him the hound was in pursuit. Soon Stapleton appears on the moor, and he barely contains his shock and disappointment when he sees the convict’s body. He says he expected to see Sir Henry, since he had invited him to come over and then heard the awful sound of the hound. Stapleton returns home, and the other two men go to Baskerville Hall, determined not to tell their host the particulars of the night’s events they have planned.

Sir Henry promises to do what he is asked without question, and Holmes is mesmerized by one of the portraits of the Baskervilles on the wall. He asks Henry if he knows who all these ancestors are, and the last of the Baskerville line tells Holmes what he has learned about his ancestry from Barrymore. Later, Holmes brings Watson back to the portrait gallery and asks if one of the men looks familiar. Watson soon sees in the face a rather stern and cold image of Stapleton, something Holmes had seen earlier in the evening. It was Stapleton, then, who followed Sir Henry in London, and it was his wife who sent the warning. This evidence that the naturalist is actually a Baskerville provides the motive behind his murder attempts on the Baskerville heirs.

In the morning, Holmes tells Sir Henry that he and Watson must leave for London but that he should attend the planned dinner at the Stapletons’ alone that night. Sir Henry is to drive over but send his carriage home and make them aware that he will be walking home. The men do not leave for London, of course, but Holmes is setting his trap. They send telegrams to Sir Henry as proof of their being in London, and then they go to visit Laura Lyons. Holmes reveals how Stapleton used her to further his own purposes and assures her that the man has no intention of marrying her because he is already married. Miss Lyons is outraged. The men leave and spend the day waiting. Watson has only a general idea of what is to come; Holmes, as always, is loath to reveal many details.

Under cover of night, the men (joined now by a representative of Scotland Yard here from London to arrest Stapleton) position themselves strategically so they can watch the Stapleton house. All is going as planned until a dense fog begins moving toward them. Soon Sir Henry leaves the house and begins his walk across the moors. He is clearly apprehensive, even frightened, but he walks. Holmes suddenly cocks his pistol and shouts that it is coming. Even though they are expecting a large dog, they are horrified and amazed by the spectral sight coming out of the fog. The lines of his face and jowls appear to be aflame:

Never in the delirious dream of a disordered brain could anything more savage, more appalling, more hellish be conceived than that dark form and savage face which broke upon us out of the wall of fog.

Sir Henry screams as the baying beast leaps at him. Sherlock Holmes empties his revolver into the animal; it rolls over and dies. Sir Henry is stunned, but the men examine the fallen creature, which appears to be a mix between a bloodhound and a mastiff. It has been painted with phosphorus to glow in the dark. Sir Henry has not recovered and is sent home; the other men now have to capture the murderer. They search the house and find Beryl Stapleton tied and gagged in an upstairs room. She is relieved to learn that Sir Henry is alive and the hound is dead. Her husband has taken refuge on the moor. His wife is ecstatic at the thought that the fog will prevent him from seeing the wands they planted together to help navigate the bogs of the Grimpen Mire.

Holmes clears up a few remaining questions as the story ends. The old missing boot is what Stapleton used to give his hound Sir Henry’s scent. (The other had been returned, of course, because it was new and did not yet carry Sir Henry’s scent.) There is evidence that Dr. Mortimer’s cocker spaniel had been eaten by the gigantic hound, which was being starved by its owner to add to its ferocity. Stapleton was undoubtedly sucked unto one of the boggy mires of the moor.

A month later, the men are again at Holmes’ Baker Street apartment discussing the case. Sir Henry and Dr. Mortimer are visiting on their way to a much-needed vacation for Sir Henry’s shattered nerves. The final revelations made by the detective include Stapleton’s motives. The naturalist was actually an unknown son of Sir Charles’s younger brother. His plans were unclear when he arrived in Devonshire, but when he heard about the family curse he began his grand scheme to gain his rightful inheritance. Although his wife was willing to be known as his sister, she was not willing to be an accessory to murder and wanted protect Sir Henry if she could. How, exactly, Stapleton planned to claim his inheritance as a Baskerville after living as someone else remains a mystery.