: The young man is accused of killing his father. Sherlock Holmes discovers that the killer is a longtime acquaintance of the victim who once robbed stagecoaches on Australia's roads.

A certain Mr. McCarthy was found killed in the Boscomian Valley. The largest landowner in the area is Mr. John Tenner, who earned his capital in Australia and returned home several years ago. He handed over one of his farms to his old friend, Mr. McCarthy, who also came from Australia. McCarthy has an only son, James, and Tener has an only daughter, Alice. The wives of both old men died long ago.

On the day of the tragedy, McCarthy left the house and headed for a small lake located in the Boscom Valley, saying that he was in a hurry for an important meeting. Soon, McCarthy met with his son, in whose hands was a gun. One of the local residents heard how the father and son quarreled, and saw how James swung at his father. Frightened, she rushed for help, but then James came running, without a gun, with blood on his hands, and said that he had found his father dead. The dead man was found near the lake, his skull was crushed by a blunt weapon, a gun was lying nearby. Police arrested James, although he denied any involvement.

The suspect said that he spent several days in Bristol.Returning, he went hunting to the lake and there he heard a cry of “Kou!” With which he and his father called each other. Father roughly asked what he was doing here, and the conversation turned into a quarrel about the reason for which he did not want to tell. James left, but soon heard a scream, returned and saw his dying father. Before his death, McCarthy muttered a few words, one was like a "rat." It seemed to James that on the ground lay something gray, like a plaid. Believing in his innocence, Mr. Tener's daughter invited Inspector Leistred for protection. Leystred hands over the case to the great detective Sherlock Holmes.

Holmes, along with his faithful friend Dr. Watson, travel to the Boscom Valley. Alice Tener tells him that the quarrel between father and son McCarthy was due to her. Mr. McCarthy wanted them to get married with James, but her father and she were against James - James is still too young.

Holmes visits James McCarthy in prison. The young man admits to the great detective that he passionately loves Alice, but several years ago he foolishly married a Bristol barmaid and no one knows about it. Therefore, he silently listened to reproaches for refusing to marry his girlfriend. However, now, having learned that James was accused of murder, the barmaid stated that she had a lawful husband for a long time, and nothing connected her with James.

The great detective draws attention to two points: McCarthy did not know when his son would return, so he had to meet with another person by the lake, and the cry “Co!” rang out before the father found out about the return of his son.

Holmes arrives at the crime scene. After examining it, he concludes that there was a third person, a tall, lame left-hander, smoking Indian cigars with a mouthpiece. The great detective finds a murder weapon - a stone. Mr. Thener turns out to be a lame left-hander: besides his son, the Australian cry "Kow!" McCarthy could only call him, and “rat” is the end of the name of the Australian state of Ballaret.

Mr. Tener confirms the conclusions of Holmes. He was a bandit in Australian mines and robbed wagons on the roads. Once, having robbed one van, he left the coachman, who was McCarthy, alive. Returning to England as a wealthy man, Mr. Tener bought the estate and began to live a quiet, honest life. To his misfortune, he met the impoverished McCarthy, who began to blackmail him. Thener did not take money from him for rent, but McCarthy began to demand that Thener marry Alice to James. Tener was not opposed to the young man, but did not want to be related to McCarthy.

Tener and McCarthy agreed to meet by the lake. Arriving at the appointed time, Tener heard how McCarthy forced his son to marry Alice, not caring how Alice herself would react to this. Out of desperation, the terminally ill Tener decided to kill.

Under pressure from the evidence presented by Sherlock Holmes, the jury acquits James McCarthy. Mr. Tener soon dies, and James and Alice live quietly in a happy marriage.