“The Adventure of the Speckled Band” begins with Watson recounting how, of the nearly seventy cases that he and Sherlock Holmes have embarked upon together as a detective duo, the one that he is about to narrate is among the most unusual of them. Watson’s telling of the narrative starts when a young woman, Helen Stoner, pays them a visit one morning, in desperate need of their help.
Helen’s stepfather, Dr. Grimesby Roylott, is the sole remaining descendent of an old family whose fortune has been slowly reduced to nearly nothing after generations of waste and gambling. When he was younger, Roylott embarked on a medical career in India, where he married the widowed Mrs. Stoner. Helen’s mother died and Roylott moved back into his family’s decaying Stoke Moran Manor, along with Helen and her twin sister Julia, all of them living in adjacent bedrooms in the building’s one inhabitable wing. Mrs. Stoner had left an inheritance to Roylott, with a stipulation that should her daughters get married, they would receive an annual income from this fund.
Two years ago, Helen tells Holmes and Watson, her sister got engaged. Shortly thereafter, Julia told Helen that she began to hear a low whistling sound in the middle of the night. This went on for some time, until, shortly before Julia’s wedding date, Helen heard a scream coming from her sister’s bedroom. Helen ran over to see what happened and Julia, in a state of shock, fell to the ground in convulsions. Julia said, “It was the band! The speckled band!” and died. The sisters were in the habit of locking their doors and shuttering their windows—Roylott keeps a wild cheetah and baboon around the property as pets—so Julia’s death fully baffles Helen.
By the time she comes to visit Holmes and Watson, Helen herself has become engaged. Soon after she makes the announcement, renovations begin on the exterior wall next to her bedroom, so Helen is forced to move into her sister’s former room, next door to Roylott. Not long after this, Helen begins to hear the same low whistling sound that Julia had described to her. Holmes and Watson then make plans to meet her at the Stoke Moran Manor later that day so they can begin investigating the mysterious incidents.
Not long after Helen leaves their apartment, a large man with a threatening demeanor enters the room. He announces himself as Dr. Roylott, Helen’s stepfather. He tells Holmes and Watson that he’s been tracing Helen’s movements, so he knows that she has just paid them a visit and he demands to know what she told them. When Holmes refuses to say anything, Roylott tells them not to meddle in his affairs. He then grabs a fire poker, bends it as show of his strength, and leaves them. Holmes remarks that he himself is stronger than he looks and he bends the poker back into shape.
Holmes and Watson journey out to Surrey later that afternoon. Meeting up with Helen, the detectives first inspect the rooms from the outside, determining that the shutters are essentially impenetrable, and then the inside, where there are a number of telling clues. Holmes examines every surface of Julia’s former room and notices that the bed is bolted to the floor, that a ventilator hole leads into Roylott’s bedroom next door, and that the bell-pull is fake (it’s merely a rope hanging onto a hook in the ceiling). They then inspect Roylott’s room, where Holmes notices a safe with a saucer of milk sitting on top of it and a leash tied and looped like a whipcord hanging from the bed.
The three hatch a plan for the evening. Holmes and Watson will take a room on the second floor of the inn across the lane from the manor. Helen will tell her stepfather that she will be confining herself to her room due a headache. When she hears that Roylott has gone to bed, she will undo the shutters in Julia’s former room, put a lamp in the window indicating that all is quiet in the house, and then retreat into her former room for the night. Holmes and Watson then go back to the inn and wait for Helen’s signal. At eleven, they see the light and head to the manor.
In Julia’s former room, Holmes instructs Watson to sit silently in the dark and to not fall asleep. They quietly wait until, hours later, they see a light coming from the ventilator hole and smell burning oil, telling them that Roylott is stirring about next door. A few moments later, they hear a long hissing sound come into their room. Suddenly, Holmes gets up and begins to furiously beat at the hanging bell-pull rope with his cane. Right when they light a lantern in their room, the glare makes it so that Watson can’t tell what Holmes had seen, only that the detective’s face had paled and a taken on a look of terror. They then hear the expected low whistling and, shortly after, a horribly loud scream coming from Roylott’s room.
Once the shrieking fades, they investigate Roylott’s room and see the doctor sitting on a chair with a snake curled tightly around his head. Holmes immediately remarks on the speckled bands around the snake’s skin and identifies it as a swamp adder, “the deadliest snake in India.” Roylott has died quickly from the snake’s lethal bite.
The story closes with Holmes telling Watson what he took away from the investigation, namely how important it is to have sufficient data, noting the confusing double meaning of the word “band,” which initially threw him off. Only by examining the physical clues directly in front of him—the bell-pull, the bolted bed and the ventilator hole—could he accurately deduce the method of the crime. Though he knows he is at least partly responsible for Roylott’s death, Holmes tells Watson, the guilt of this evil man dying will not weigh on him very heavily.