The book begins with a young girl's suicide by leaping off the newly built Brooklyn Bridge. Sherlock Holmes, under the alias of Simon Hawkes (he's on the run from Moriarty's gang) is living in an apartment in 1893's New York City's lower East Side, in a men's club called "The Dead Rabbits Society". Subsequent to the girl's death (which initially seems unrelated to subsequent events) a man named Frank Dunmore, also a member of the "Dead Rabbits" comes to Hawkes (Holmes) and declares that someone is trying to kill him!
Not one but two attempts have been made on his life, first a strangling attempt and then a bullet through his window as he sat reading a book. The small nervous man is scared half to death. Hawks agrees to look into the matter and with that is drawn into a case of cold-blooded murder in which nothing is what it appears to be on the surface. Another attack on Dunmore takes place, he is shot at while at a dinner table with friends! And this time an innocent bystander, a Mrs. Hammand, a staunch fighter for Women's Rights, seated next to Dunmore, is shot to death!
At first the case is deep and dark, the New York police are stymied, and even Hawkes/Holmes, with his amazing powers of deduction, cannot initially fathom the truth but slowly, using his amazing powers of deduction, Hawkes brings the true killer to light. But knowing who the killer is and bringing the killer to justice are two separate things. Will the killer pay for the crimes committed? Very unusual ending for a Holmes novel, but one that is true to the Doyle character.