The three detectives approach Merripit House, and Holmes insists that they all tiptoe so they are not heard. Hidden behind some rocks, the group observes Sir Henry and Mr. Stapleton chatting over coffee. Sir Henry seems nervous, perhaps pondering the long walk home across the moor.
Just then, Stapleton gets up and heads outside, letting himself into a small outhouse where the hidden group hears some strange scuffling. Meanwhile, a thick fog starts to settle and spread across the moor, and the group gets nervous as the visibility gets worse and worse. Once the fog engulfs the path from Merripit to Baskerville Hall, the detectives will not be able to watch Henry's walk home, nor protect him when the hound attacks.
Once Henry finally gets going, the fog covers the path, and the detectives hear the hound before they see it. When it emerges from the mist, the hound turns out to be an immense, iridescent, fire-breathing beast, the very picture of the Baskerville myth. Stunned, the detectives only shoot one round of bullets as the hound nips at Henry's heels. But the shots do not kill the beast, and it leaps at Henry's throat. Fortunately, Holmes manages to unload five more rounds at just the right moment, and the hound collapses.
Examining the baronet, they discover no injuries. Getting a chance to finally examine the animal, the detectives determine it to be a bloodhound-mastiff mix, as big as a lion and covered with phosphorous to make it glow. Rushing back to the house, the detectives discover Mrs. Stapleton bound and gagged.
Waking up, Mrs. Stapleton makes sure Sir Henry is safe and the hound is dead, and then informs the detectives of her husband's hiding place in the Grimpen mire, the deadly marshland where he kept his hound. Deciding that the fog is too thick to pursue the villain through the treacherous mire, Holmes and Watson head back to Baskerville Hall with Sir Henry.
The next day, Mrs. Stapleton leads them through the mire, eager to capture her abusive husband. The Stapletons had placed sticks in the mire to mark the spots where it was safe to walk, and the detectives follow the path until they come upon an object, partially submerged. It turns out to be Sir Henry's black boot, which Stapleton used to set his hound on Henry's trail and then threw to the ground as he made his escape. As for Stapleton himself, his footprints are nowhere to be found beyond a certain point, and the detectives decide that the great Grimpen mire has engulfed him. When they reach his lair, they discover the place where the hound was kept, hidden away but still audible for miles around. The villain brought his hound to Merripit only that last day, so dangerous was the risk of discovery. The detectives also find the phosphorous used to make the beast glow—scary enough to frighten Sir Charles to death.
Back in London, Henry and Mortimer call on the detectives to get the full rundown of the confusing case. Holmes explains that Stapleton was actually the son of Roger Baskerville, Charles' younger brother who moved to South America and was presumed dead. Stapleton, or Sir Roger Baskerville, Jr., lived in South America and married Beryl Garçia of Costa Rica, the dark and lisping beauty masquerading as his sister. Having embezzled public money, Roger fled to England, changed his name, and established a school up north. When the school folded, Roger had to take off again, this time heading to Devonshire where he had heard of his stake in a large inheritance. Having made friends with Sir Charles, Roger heard of the myth of the hound and of Charles' bad heart.
To get the superstitious Charles out alone on the moor, Stapleton tried to enlist his wife, but she refused. He happened, however, to meet Laura Lyons, and he told her he would marry her if she got a divorce. Convincing her to get the necessary money from Charles, he made her miss the late-night appointment and unleashed his hound. Though Laura suspected Stapleton, she protected him out of love.
Once Henry arrived on the scene, Stapleton took his untrustworthy wife with him to London, where he trailed the baronet and she tried to warn him. Stapleton also made a point of stealing one of Henry's shoes to give his hound the baronet's scent. But the first boot he stole was brand new, not yet worn by Sir Henry and unsuitable for its intended purpose.
Holmes mentions that Mrs. Stapleton's letter smelled of perfume, and that the suggestion of a gentlewoman made him think right from the start of the Stapletons. Going on to investigate and ultimately establish Stapleton as the enemy, Holmes nonetheless needed proof, so he used Henry as bait to catch Stapleton red-handed. Holmes apologizes for using the baronet, but insists that it was necessary.
Mrs. Stapleton, for her part, both loved and feared her husband, and she was willing to warn Henry but not to reveal her husband's involvement. Stapleton himself encouraged the romance but could not help a jealous outburst the day he saw the two talking intimately.
On the night Henry came to dinner, Mrs. Stapleton realized her husband had his hound in the outhouse, and she confronted him. He revealed his relationship with Laura, and, when she reacted, he tied her up and gagged her. The only other loose end, as Holmes sees it, is just how Stapleton intended to claim the fortune. Though Holmes speculates that perhaps he would claim it from South America, he admits that he cannot predict behavior in the future. Henry heads off for a vacation with Mortimer to calm his nerves.
When the detectives finally encounter the hound it is not enough that he is glowing and breathing fire, he has to emerge out of a thick fog. Doyle's whole Gothic apparatus, the themes of fantasy and the supernatural, the curse, the manuscript, the manor, all of it has led up to this one moment, when the hound leaps out of the hazy world of imagination and into the detectives' realm of reality. It is a key moment of climax. After the action has subsided, it is really only after they have killed the hound that the detectives get a good look at him. Once again, the detectives encounter a kind of disguised identity, discovering the artifice that made the hound look supernatural. The juxtaposition of the plot-driven climax of the hound's appearance and the thematic climax of its unmasking clearly reveals the ways in which Doyle uses a kind of Gothic, folk tale tradition in service of his story. In the end, mystery is exciting but closure is comforting.
In "A Retrospection," Holmes gives us all the comfort we need and a synopsis of the entire story. He ties up all the loose ends and even claims to have known right from the start that the Stapletons were the ones to blame. Interestingly, though, the wrap up is not that neat, with Henry headed off to calm his nerves on a vacation. Henry and Beryl do not get married and live happily ever after, and it is not even clear that Stapleton is actually dead. It has been suggested that Doyle considered bringing Stapleton back in a later story, but "what a man may do in the future is a hard question to answer."