Most Sherlock Holmes stories are solved with the same formula. Holmes and Watson are usually sought out by someone in distress who tells them of their problem. Then the duo set out to investigate. In this story, Helen Stoner seeks out Holmes and he and Watson soon arrive to the estate to investigate the death of her sister two years earlier. The only thing to go on is that the sister said the words, "...the speckled band..." right before she died. So what was the speckled band?
At the estate Holmes notices a few details, as usual he tends to see what most others have overlooked. Firstly, he remarks that Helen is being forced to move into the room where her sister died indicating to Holmes that there must be something special about this room and that the murderer might be ready to strike again. Therefore Holmes investigates the room and find several odd things. For one the bed is nailed down. By assessing the peculiarities of the room, he is able to put together a plausible course of action.
In the end, Holmes catches the step-father in the act. He has used a bell cord and the ventilation system which connected his room with the step daughter's to let down a poisonous snake which was intended to bite her in her sleep. The snake itself is the 'speckled band' to which Helen's sister referred to just before she died.