Helen Stoner is so afraid because her twin sister Julia died under mysterious circumstances at their home two years previously, and now she fears her turn is next. Their home is a grim and gloomy place owing to the presence of their stepfather, Dr Roylott, who is a fearsome, violent man, and Julia died suddenly in her own bedroom one night. There is no real clue as to what happened, but Helen found her in extreme terror, and babbling something about 'a speckled band'. A few nights before she died, Julia had told her about hearing a strange whistle at night. Just recently, Helen has had to move into her sister's old room where she died, and she too has heard the strange whistle. Helen is terrified that this whistle heralds her death, and so she has come immediately to consult Holmes.
Helen arrives 'in a pitiable state of agitation', as Watson sympathetically notes. She is wearing a black veil which increases the whole air of foreboding that hangs around her. Holmes adopts a soothing manner and helps to calm her fears. Of course, Holmes also goes on to solve the mystery of her sister's death, in which Dr Roylott is revealed as the culprit. Roylott has a deadly pet, a snake which he trained to climb into Julia's room at night, and which he used to summon back by a whistle - the whistle that Julia and Helen had heard. The 'speckled band' referred to the apprearance of the snake. Helen, however, has saved herself by coming to ask Holmes's help, and the snake ends up killing Dr Roylott, so that he is justly punished.
Her sister had died, and her last words were the "specled band", so she became afraid for her own life, from whom, she does not know, however Sherlock Holmes uncovers the plot.