Summary of The Boscombe Valley Mystery

Lestrade summons Holmes to a community in Herefordshire, where a local land owner has been murdered outdoors. The deceased's estranged son is strongly implicated. Holmes quickly determines that a mysterious third man may be responsible for the crime, unraveling a thread involving a secret criminal past, thwarted love, and blackmail.

Sherlock Holmes and Dr. Watson take a train to Boscombe Valley, in Herefordshire. On the way, Holmes reads the news and briefs Watson on their new case.

Mr. John Turner, a widower and a major landowner who has a daughter named Alice, lives there with a fellow expatriate from Australia, Mr. Charles McCarthy, a widower who has a son named James. Charles has been found dead near Boscombe Pool. It was reported that he was there to meet someone. Two witnesses testify that they saw Charles walking into the woods followed by James, who was bearing a gun. Patience Moran, daughter of a lodge keeper, says she saw Charles and James arguing and, when James raised his hand as if to hit his father, she ran to her mother. While telling her mother what she saw, James rushed to their house seeking help. The Morans followed James back to the Pool, where they found his father dead. James was arrested and charged with murder. Alice Turner believes James is innocent and has contacted Lestrade, a Scotland Yard detective who in turn has asked Holmes’ help.

James confirms the testimonies of the witnesses, but explains that he went to the woods to hunt, not to follow his father. He later heard his father calling "Cooee", and he found his father standing by the pool, surprised to see him. They argued heatedly, and James decided to return to Hatherley Farm. Shortly thereafter, he heard his father cry out, and returned to find his father lying on the ground. James insists that he tried to help him, but his father died in his arms. James refuses to reveal the cause of their argument, despite the coroner's warning that it could be damaging to his case. James also remembers his father’s last words were something about "a rat", but James is uncertain of the meaning. He also saw a cloak nearby that was gone when he returned later.

Alice meets Holmes, Watson and Lestrade in the hotel; she hopes Holmes has found a way to prove James' innocence. She also believes that she was the subject of the argument between James and his father, for Charles had asked James to marry her but James refused. Alice's father was also against the union. Holmes asks Alice if he could meet her father, but she says his health worsened after the death of Charles, whom he had known since they were in Victoria. Holmes decides to see James.

Holmes mistakenly surmises that James knows who killed his father and is only protecting someone. Alice is right about the cause of the argument between James and Charles. What she does not know is that James loves her and wants to marry her, but could not because he had already married a barmaid before Alice returned from boarding school. This burdens him, but he cannot tell his father about his marriage because he would be thrown out of the house and left unable to support himself. To James’ consolation, when his wife hears of his troubles, she confesses she was already married before they met, and therefore their marriage is invalid.

Holmes, Watson and Lestrade go to Hatherly Farm and examine Charles' and James' boots. They then head to Boscombe Pool, following the track from the courtyard. After examining the ground, Holmes finds evidence of the presence of another man besides Charles and James, whom he believes to be the murderer. The stranger is a tall, limping, left-handed man who smokes cigars. Lestrade is not convinced.

At the hotel, Holmes explains to Watson that "Cooee" is an Australian cry and "a rat", overheard by James, were the last syllables of "Ballarat", a place in Australia. So the person Charles was meeting is someone he knew from Australia. John comes to their room and, realizing that Holmes has deduced the crime, confesses.

In his confession, John explains that he was a member of the Ballarat Gang, a group of robbers in Australia. They robbed a gold convoy in which Charles was the wagon driver, and John spared his life despite knowing that Charles could identify him. The loot made the gang rich, and they moved to England. Resolved to change, John parted ways with his friends. He bought land, married, and then Alice was born. John met Charles again by chance, and Charles threatened to blackmail him. In response John gave Charles Hatherly Farm and money. Eventually this was not enough, and Charles demanded the marriage of James and Alice. Although he likes James, John could not allow Charles eventual control over his family's finances through Alice, and resisted the union. After much pressure, John agreed to meet Charles secretly at the Pool. Seeing Charles and James there arguing, John waited till James left. Then he killed Charles to preserve his freedom and spare his daughter. James heard his father's death cry and returned, but John was able to hide in the woods. He had to return later to retrieve a cloak that he dropped in his haste.

John signs his statement, and Holmes vows to keep it secret unless needed to free James.

Holmes' objections are sufficient to acquit James. John dies seven months after the meeting with Holmes and Watson. A brighter future is ahead for James and Alice, free of their fathers' dark past.