What is "The Adventure of The Speckled Band" mainly about?

Sherlock Holmes calls himself a "consulting detective." Nowadays we might call him a private detective, a private eye, or even a shamus. He is strictly a non-professional, and as such he is limited as to the kinds of cases he can take on. Then as now the police do not want private detectives interfering with murder cases. Holmes is able to get involved in solving murders only because he has done innumerable favors for Scotland Yard detectives and has allowed them to take all the credit for solving many cases he has solved himself. But it should be noted that Holmes rarely gets involved in murder cases before the police have been summoned to the scene of the crime. In the best-known Sherlock Holmes tale "The Hound of the Baskervilles," for example, Holmes is not trying to solve a murder but to prevent a murder. This is strictly legitimate work for a private eye, and many of the Sherlock Holmes stories are about clients who are in danger of being killed but are not actually dead.

In "The Adventure of the Speckled Band," Helen Stoner comes to Holmes for advice. She thinks she might be in mortal danger and is terrified. Holmes goes down to Stoke Moran accompanied by Watson, not to investigate the two-year-old death of Julia Stoner, but to protect Helen. It is noteworthy that he does not even try to arrest Dr. Roylott when he discovers what the vicious man has been up to; instead, the author conveniently arranges for Roylott to be killed by his own snake. Holmes simply does not have the authority to make an arrest. He does not even make the truth about Roylott's swamp adder or about Julia Stoner's death publicly known. Dr. Watson explains in the opening paragraph that the whole thing has been kept secret until he is currently revealing the truth in this story he entitles "The Adventure of the Speckled Band."

It is possible that I might have placed them upon record before, but a promise of secrecy was made at the time, from which I have only been freed during the last month by the untimely death of the lady to whom the pledge was given. It is perhaps as well that the facts should now come to light, for I have reasons to know that there are widespread rumours as to the death of Dr. Grimesby Roylott which tend to make the matter even more terrible than the truth.