You're in luck! eNotes has summaries of lots of books and short stories available to students. Here is a link to a summary of The Speckled Band by Sir Arthur Conan Doyle. I'm happy to give you a short summary here as well.
The Adventure of the Speckled Band takes place relatively early on in the partnership of Mr. Sherlock Holmes and Dr. John Watson. Early in the morning, Watson is awoken by Holmes, who tells him they have a patron waiting to share her case with them. A young woman is waiting in the sitting room and tells them that she is beginning to fear for her life.
Miss Helen Stoner lives with her stepfather, Dr. Grimesby Roylott, on a large estate in the countryside. Dr. Roylott used to live and work in India, where he met and married Helen's mother. Several years prior, Helen's mother died in a mysterious accident officially determined a train accident. More recently, Helen's dear sister Julia passed away, too. It is because of the events of Julia's death that Helen has come to be so fearful. Before her demise, Julia was recently engaged. In the middle of the night, she would hear strange whistling and clanging, which Helen swore she could not hear.
Now than Helen has moved into Julia's old bedroom-- some renovations are being done on her own room-- she has been hearing the strange noises her sister spoke of before she passed. Helen, too, is recently engaged, and has come to Mr. Holmes and Dr. Watson because she feels the circumstances are too similar to ignore.
Holmes and Watson travel to the estate of Dr. Roylott and Miss Stoner to investigate. They offer to sleep in Julia's old bedroom for a night while Helen stays at a nearby inn. During the night, they hear the clanging of a grate above the bed, which Holmes strikes with his walking stick. When they emerge from the bedroom, they find Dr. Roylott dead on the other side.
It turns out that Dr. Roylott has been trying to send a poisonous snake, brought back from India, through the grate in Julia's wall to attack Helen in the night. He presumably did just the same to Helen years prior. Holmes determines that Dr. Roylott has been picking off the women in his life to prevent the family's fortune from being spread out. If Helen or Julia were to marry, the grand inheritance left behind by their mother would have passed on to them.
It is out of greed that Dr. Grimesby Roylott tried to kill his stepdaughters, and yet he found himself bitten by his own pet snake.