Summary
James McCarthy, the son of Charles McCarthy, is accused with the murder of his father, who was found beaten to death by the Boscombe Pool, a small lake near their home on the land of James Turner, an acquaintance of Charles. All the evidence seems to point towards James' guilt, since he was carrying a gun and had a violent altercation with his father just before he was discovered with the body, but Holmes suspects that something else is at play.
After detailed investigation of the scene and consultation of Charles' autopsy in the coroner's notes, Holmes determines that it was in fact James Turner who had killed Charles. Turner admits his guilt, saying that Charles was an evil character who was blackmailing him with knowledge of crimes they committed together when they were in Australia so that he could live on his, Turner's, land.
Analysis
One of the main clues in this mystery is the victim's last words, which his son hears as "a rat." Holmes, ever the purveyor of obscure pieces of knowledge, realizes that this must be part of a name and, learning of McCarthy's and Turner's past in Australia, determines that "ARAT" was actually the ending of "Ballarat," an Australian town. John Turner himself admits to having taken the alias "Black Jack of Ballarat" during his criminal days there. Since the mysteries that Holmes solves most often depend upon the identification of a specific perpetrator, they often take the form of a hunt for a specific name; in this case that name is almost given and eventually, serving as a kind of synecdoche for the entire investigation, is revealed in full.
The marriage ending of this story is also remarkable in its moral ambiguity. "There is every prospect that the son and daughter may come to live happily together, in ignorance of the black cloud which rests upon their past," (103) we learn from Watson. Since both criminal fathers have died and James McCarthy has been acquitted of the crime, it is as though all has been solved; however, (only) Holmes and Watson know of the unpleasant foundations on which this future is laid.
