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.