As Holmes, Watson, and Lestrade drive over to Merripit House, the suspense is killing Watson.
Holmes and Lestrade hide about two hundred yards away from the house.
Holmes sends Watson to spy through the dining-room window.
Sir Henry and Stapleton are sitting together and smoking.
Beryl's nowhere to be seen.
The Grimpen Mire is covered with fog, which is a bummer for Holmes—fog is the one thing that could really endanger Sir Henry's life.
They hear the sounds of Sir Henry leaving the house.
And then, Holmes hushes Watson—there's another sound of pattering feet coming.
It's a black hound covered in flickering flame, with fire coming out of its mouth.
Oh no, it's running toward Sir Henry.
Holmes and Watson shoot the dog, which howls but keeps running.
Sir Henry is looking behind him at the dog—and he looks terrified. Duh. The dog leaps at Sir Henry and starts biting him. Bad dog!
But Holmes catches up and empties his gun into the dog.
The dog falls dead.
Sir Henry faints, but he's still alive.
When Sir Henry comes to, he, Holmes, and Watson inspect the body of the dog.
It's an enormous beast with huge jaws, and it's been covered in some kind of weird glow-in-the-dark stuff.
Watson touches the stuff on the dog's fur and realizes that it's phosphorus (a glowing chemical).
Holmes apologizes for putting Sir Henry in so much danger—he didn't expect either the fog or the dog.
Sir Henry's so freaked out that Holmes and Watson leave him sitting on a rock while they go off after Stapleton.
Back at Merripit House, there's a locked bedroom door.
Holmes breaks down the door. They find a woman bound and gagged: Beryl Stapleton.
She's furious and heartbroken that Stapleton has been abusing her and using her as his tool in his schemes against Sir Henry.
Beryl says that Stapleton has a hiding place in the middle of the Grimpen Mire. The fog is so dense that he won't be able to leave his hiding place that night.
The next morning, Beryl leads Holmes, Watson, and Lestrade through the dangerous bog.
As the three men walk deep into the Grimpen Mire, Holmes spots something: Sir Henry's black boot.
Stapleton must have been using the boot to teach the hound to track Sir Henry's smell.
But they don't find any other sign of Stapleton.
Watson believes that Stapleton probably got lost in the fog that night and fell into the Mire, never to emerge.
On the island in the Mire, they find traces of the dog: this must be where Stapleton kept it.
Sadly, they also find the skeleton of Dr. Mortimer's little spaniel.
There's a pot full of the glowing stuff that Stapleton had been using to create the fire-breathing "Hound of the Baskervilles," which frightened Selden into running over a cliff and scared Sir Charles to death.