A young woman named Helen Stoner consults the detective Sherlock Holmes about the suspicious death of her sister, Julia. One night, after conversing with her twin sister about her upcoming wedding day, Julia screamed and came to the hallway where Helen came out to see her, in Julia’s dying words she said “it was the band, the speckled band! ” Julia had been engaged to be married and, had she lived, would have received an annual GBP250 annuity from her late mother’s income. Now Helen is engaged to be married.
Holmes’ investigation of the mother’s estate reveals that its value has decreased significantly, and if both daughters had married, Dr. Roylott, Helen’s ill-tempered and violent stepfather, would be left with very little, while the marriage of even one would be crippling. Therefore, the main suspicion falls on him. Dr. Roylott has required Helen to move into Julia’s old room of his heavily mortgaged ancestral home, Stoke Moran. A number of details about the place are mysterious and disturbing. A low whistling sound is heard late at night, as well as a metallic clang.
There is a strange bell cord over the bed, and it does not seem to work any bell. The rope goes to a ventilator to the step father’s room. The bed is also unusually clamped to the floor so that it can never be moved from its position. Stoner surmises that Julia might have been murdered by the gypsies, whom Dr. Roylott permits to live on the grounds of the home;they wear speckled handkerchiefs around their necks. A cheetah and a baboon also have the run of the property, for Dr. Roylott keeps exotic pets from India. Helen feels reluctant to sleep in the room.
After Helen leaves, Dr. Roylott comes to visit Holmes, having traced his stepdaughter. He demands to know what Helen has said to Holmes, but Holmes refuses to say. Dr. Roylott bends an iron poker into a curve in an attempt to intimidate Holmes, but Holmes is unaffected as he maintains a rather jovial demeanor during the encounter. After Roylott leaves, Holmes straightens the poker out again, thus showing that he is just as strong as the doctor. Having arranged for Helen to spend the night in her original bedroom, Holmes and Watson sneak into her bedroom without Dr.
Roylott’s knowledge. Holmes says that he has already deduced the solution to the mystery, and this test of his theory turns out to be successful. They hear the whistle, and Holmes also sees what the bell cord is really for, although Watson does not. Julia’s last words about a “speckled band” were in fact describing “a swamp adder, the deadliest snake in India”. The venomous snake had been sent to Julia’s room by Dr. Roylott through the ventilator to murder her. The fake bell cord is to act as a bridge for the snake to land on the bed.
After the swamp adder bit Julia, he called off the snake with the whistling, which made the snake climb up through the bell cord, disappearing from the scene. Now the swamp adder is sent again through the ventilator by Dr. Roylott to kill Julia’s sister Helen. Holmes attacks the snake, sending it back through an air ventilator connected to the next room. The aggravated snake bites Dr. Roylott instead, and, within seconds, he is dead. Holmes grimly notes that he is indirectly responsible for Dr. Roylott’s death, but that he is unlikely to feel much guilt over the death.