The novel starts with Doctor Mortimer bringing a case to Sherlock Holmes—the case of the Hound of the Baskervilles. And since Doctor Mortimer is so sure that the devil's involved in this murder mystery, we might be forgiven for thinking that the "Monster" that Holmes has to overcome in this book is a huge hell beast, demonically set on destroying the Baskerville family. However, that sounds more like a Stephen King novel than a Sherlock Holmes story—there must be some other, more human monster at work in the death of Sir Charles Baskerville and the threats against his heir, Sir Henry.
While Sir Henry Baskerville is still in London throughout the first five chapters of the novel, there are certainly a lot of suspicious happenings. A lot of facts and theories are floating around. Something is obviously going on, and he's at the heart of it. But we're still pretty far from the actual scene of the crime, Baskerville Hall. In spite of the disappearing boots, the warning letter, and the man following Sir Henry and wearing a ridiculously large false beard, we still can't really see the shape of the threat facing Sir Henry. A Hound from hell still seems as good an explanation as any for what happened to Sir Charles Baskerville.
Once Watson and Sir Henry get to Baskerville Hall in Chapter 6, things take a darker turn. The plot thickens with Barrymore the butler, Selden the escaped convict, Beryl Stapleton the love interest, and Laura Lyons the mystery acquaintance of Sir Charles'. This all combines to give us the sense that someone's plotting behind the scenes to destroy Sir Henry, even if we can't quite see who it is quite yet. What's more, the generally gloomy atmosphere of the setting at Dartmoor contributes to this overall sense of threat but without any resolution in sight.
Once Holmes arrives in Chapter 12, we finally know for sure whodunit: it's Stapleton, the local scientist and fraud. The threat now comes into focus. However, just because we know who the Monster is does not mean that Holmes is ready to overcome him. They have no evidence against Stapleton that would stand up in court. So Holmes sets up a pretty risky ambush, using Sir Henry as bait. Between the thick fog and the sudden appearance of a huge glow-in-the-dark dog, Holmes almost manages to let Sir Henry get killed after all.
The Thrilling Escape from Death, and Death of the Monster
Of course, since Holmes never fails too badly, he does manage to shoot the dog before it kills Sir Henry. Watson finds that the dog's fur has been covered with phosphorus to make it glow, which is why it looks like the ghost dog from hell. Stapleton escapes into the fog, but he runs straight into the Grimpen Mire, a dangerous bog in which he probably drowns. So all's well that ends well: the vicious dog turns out to have been trained by an evil human, rather than by the devil. And that human—Stapleton—has paid for his crimes with his life. Holmes the Great Detective does it again and the universe is back in balance.