A lady named Helen Stoner visits Sherlock Holmes to ask for his help as her sister had just died under mysterious circumstances. She had a stepfather named, Dr. Gamersby Roylott who was one of the last surviver of the roylotts of stoke moran, on the Western border of surey. He married to Mrs Stoner , the young widow of the major general stoner of the bengal Artillery. She also had a twin sister named Julie. Dr. Roylott was a very short tempered and dominating stepfather. Mrs. Stoner died when the two twins were just 8 years old and she had told to offer them a considerable sum of amount in their weddings,not less than a thousand pounds a year and this she beaqethed to Dr. Roylott. He had a passion of keeping Indian animals which were sent to him by a correspondent . He also smoked Indian cigars. 

One day before the wedding night, both the sisters were sitting in helen's room discussing about Julie's approaching wedding. Julie could smell the strong Indian cigars which he smoked. Julie asked Helen that could she also here the low, clear  whistle at the dead of the night? Helen said no, she thought that it would be those wretched gypsies in the plantation, who wonder freely over their grounds. Julie left Helen's room and went to sleep. At the mid of the night, Helen heard a scream of a terrified women. She knew that it was her sister. She spranged from her bed ,wrapped a shawl around her and ran down the corridor. Her sisters door was unlocked and it revolved slowly upon its Hinges. She was in a pathetic state, her face was all drawn Gray, her limbs were dreadfully convulsed and was moving like a drunkard. She suddenly shrieked out to Helen that it was the band, the speckled band. Such was a dreadful end of her beloved sister, julie. Dr, Roylott did this just to have the money and as he was a stepfather he had no respect for them. 

Sherlock Holmes went to stoke moron, and examined the house and Her room as Dr. Roylott had went to the city. Helen's room was willingly destroyed as he wanted to make Helen shift into her sisters room and kill her the same way as it was also her one day before wedding. At night Sherlock Holmes arrived and was sitting and examining the room with Dr Watson, as Helen had told him to help her out. There was a ventilator between dr Roylott and julie's room which was seen by Sherlock Holmes. The night when Julia died Dr Roylott had trained the snake with the help of a whistle and sent him through the ventilator to kill Julie. But this time  Sherlock Holmes saw it and it returned and killed Dr. Roylott. 

So,  it was a dreadful snake that killed Julie and was known as the speckled band.