Exposition (Initial Situation)
Here, Doggie! Nice Doggie! (Please Don't Eat the Baskervilles)
At the start of The Hound of the Baskervilles, detective Sherlock Holmes and his loyal friend Doctor John Watson are sitting happily in the apartment they share (as roomies) when Doctor James Mortimer arrives to tell them a bizarre family legend about a giant demon dog that haunts the Baskerville family in the southwestern English region of Dartmoor. In fact, according to Doctor Mortimer, this ghostly Hound has recently added another victim to its list: Sir Charles Baskerville, whose dead body was found lying in his driveway with a grotesque expression of fear on his face.
Doctor Mortimer is so sure that the Hound is responsible for Sir Charles' death that he doesn't think it's worthwhile for Holmes to investigate further. He just wants to know what Holmes thinks he should tell the new heir to Sir Charles' estate, Sir Henry Baskerville. After all, he wants Sir Henry to be comfortable in his new home—and he doesn't want to make Sir Henry another victim of the Hound's appetite for Baskervilles. So, the initial situation is one of danger and the supernatural: Doctor Mortimer believes that this is a ghost story, even though Holmes is a supremely practical and rational detective.
Rising Action (Conflict/Complication)
Maybe the Murderer Isn't a Hound From Hell After All?
We spend Chapters 4 through 12 of this novel picking up about a million plot twists (only a slight exaggeration). Obviously, Holmes does not buy the ghost Hound story. He's a scientific and logical detective, after all. But there is something weird going on at Baskerville Hall, and it seems to center on poor Sir Henry, the new heir. So when Sir Henry makes plans to return to his old family home, Holmes asks Watson to go with him as a kind of bodyguard/investigator combo. And in the background of all of the strange happenings that surround Sir Henry, there's always the rumor of the Hound creeping out both him and Watson. Who—or what—is threatening Sir Henry's life? And how can we figure it out without the help of Sherlock Holmes, the greatest detective who ever lived?
Climax (Crisis/Turning Point)
It's Hard to Take a Villain Seriously When He's Always Carrying a Butterfly Net
Well, all the many mysteries swirling around Baskerville Hall get (mostly) solved in Chapter 12, when Sherlock Holmes finally reappears on the scene. Holmes tells Watson that there's no question that Stapleton, the local naturalist, is also our killer.
But while Holmes is sure that Stapleton is the murderer of Sir Charles Baskerville, he has no proof that would stand up in court. How do you prove something so farfetched: that Stapleton used a giant dog to frighten to death an old man with a heart condition? And how are they going to catch Stapleton in the act of doing whatever he's planning to do to Sir Henry? Holmes' happy reunion with Watson ends the mystery portion of the novel, since we know the main secret of the plot—the identity of the murderer—but we still have to wait for Holmes to make his move against Stapleton once and for all.
We'll Show Him!
We spend Chapters 13 and 14 watching Holmes set up his final ambush of Stapleton. He and Watson talk to Laura Lyons, confirming that Stapleton demanded she set up a meeting with Sir Charles, which he then persuaded her not to attend. What's more, Stapleton bullied Laura into keeping quiet about the meeting after Sir Charles' mysterious death. Once Holmes sets up Sir Henry as bait by sending him off to dinner with Stapleton that night, they're ready to catch Stapleton red-handed in an attack on Sir Henry.
Murder By Dog
In Chapters 14 and 15, Conan Doyle finally ties up all of the loose ends of the novel: we discover that Stapleton's dog is a huge animal that he's been keeping on an island in the middle of a dangerous bog called the Grimpen Mire. He's been covering the dog with phosphorus to make it glow in the dark, which is what sent Sir Charles into cardiac arrest and sent Selden running over that cliff.
Stapleton sends the dog to attack Sir Henry the night that Holmes and Watson are watching for him, but Holmes manages to shoot it before it can do Sir Henry too much damage (except, of course, scaring the bejeezus out of him). Stapleton disappears into the fog, trying to find a place to hide in the Grimpen Mire. But Watson guesses that Stapleton may have taken a wrong step in the bog and drowned, since he never emerges from the Mire.
Later, Holmes explains that Stapleton wanted to take advantage of the Baskerville family legend because he was actually the long-lost son of Sir Charles' dead brother Rodger Baskerville, who disappeared into South America to escape some trouble in England. Stapleton's claim to the Baskerville fortune is therefore a pretty good one—except for the two people, Sir Charles and Sir Henry, who stood in his way.
Stapleton's threat to Sir Henry has been resolved thanks to Sherlock Holmes' brilliance and the dangerously soft ground of the Grimpen Mire. All has been explained logically without a single ghost, and Watson and Holmes are back in London and their rational world.