Frank Osbaldistone is recalled from France, where his father had sent him to learn the family’s mercantile business. Disappointed in his son’s progress, the father angrily orders the young man to Osbaldistone Hall, home of his uncle, Sir Hildebrand Osbaldistone, in northern England. His father gives him fifty guineas for expenses and instructions to learn who among Sir Hildebrand’s sons will accept a position in the trading house of Osbaldistone and Tresham.

On the road, Frank falls in with a traveler named Morris, who is carrying a large sum of money in a portmanteau strapped to his saddle. That evening, they stop at the Black Bear Inn in the town of Darlington, where they are joined at dinner by a Scotsman named Mr. Campbell, who is really Rob Roy, the Scottish outlaw. The next morning, Campbell and Morris leave together. At a secluded spot along the road, the men are halted and a highwayman robs Morris of his saddlebag. Meanwhile, Frank rides toward Osbaldistone Hall. As he nears the rambling old mansion, he sees that a fox hunt is in progress and meets Diana Vernon, Sir Hildebrand’s niece. The outspoken Diana tells Frank that all of his cousins are mixtures in varying proportions of sot, gamekeeper, bully, horse jockey, and fool. Rashleigh, she says, is the most dangerous of the lot, for he maintains a private tyranny over everyone with whom he comes in contact. It is Rashleigh, however, who is prevailed upon to accept Frank’s vacant position at Osbaldistone and Tresham.

Frank and his cousins dislike one another. One night, while drinking with the family, Frank becomes enraged at Rashleigh’s speech and actions and strikes him. Rashleigh never forgets the blow, although to all intents and purposes he and Frank declare themselves friends after their anger has cooled.

Shortly after Frank’s arrival at Osbaldistone Hall, he is accused of highway robbery. He goes at once to Squire Inglewood’s court to defend himself and to confront his accuser, who turns out to be Morris. Rob Roy, however, appears at the squire’s court of justice and forces Morris to confess that Frank is not the man who robbed him.

When Rashleigh departs to go into business with Frank’s father, Frank becomes Diana’s tutor. Their association develops into deep affection on both sides, a mutual attraction marred only by the fact that Diana is a Catholic and Frank a Presbyterian.

One day, Frank receives a letter from his father’s partner, Mr. Tresham. The letter informs him that his father has gone to the Continent on business, leaving Rashleigh in charge; Rashleigh, however, has gone to Scotland, where he is reportedly involved in a scheme to embezzle funds from Osbaldistone and Tresham.

Frank, accompanied by Andrew Fairservice, Sir Hildebrand’s gardener, sets off for Glasgow in an attempt to frustrate Rashleigh’s plans. Arriving in the city on Sunday, they go to church. As Frank stands listening to the preacher, a voice behind him whispers that he is in danger and that he should not look back at his informant. The mysterious messenger asks Frank to meet him on the bridge at midnight. Frank keeps the tryst and follows the man to the Tolbooth prison. There he finds his father’s chief clerk, Mr. Owen, who has been arrested and thrown into prison at the instigation of MacVittie and MacFin, Glasgow traders who do business with his father. Frank learns that Campbell is his mysterious informant and guide, and, for the first time, he realizes that Campbell and Rob Roy are one and the same.

Shortly thereafter, Frank sees Morris, MacVittie, and Rashleigh talking together. He follows them, and, when Morris and MacVittie depart, Frank confronts Rashleigh and demands an explanation of his behavior. As their argument grows more heated, swords are drawn, but the duel is broken up by Rob Roy, who cries shame at them because they are men of the same blood. Rob Roy considers both men his friends. Frank also learns that his father’s funds were mixed up with a Jacobite uprising in which Sir Hildebrand was one of the plotters. He suspects that Rashleigh robbed Morris based on information supplied by Rob Roy.

Frank and Andrew are arrested by an officer on their way to meet Rob Roy, and the officer who searches Frank discovers a note that Rob Roy had written to him. On the road, the company is attacked by Scotsmen under the direction of Helen, Rob Roy’s wife, who captures or kills all the soldiers. Helen, a bloodthirsty woman, orders that Morris, who has fallen into the hands of the Highlanders, be put to death. In the meantime, Rob Roy too is captured, but he makes his escape when one of his captors rides close to him and surreptitiously cuts his bonds. Rob Roy then throws himself from his horse into the river and swims to safety before his guards can overtake him.

With a Highland uprising threatening, Frank thinks he has seen Diana for the last time, but he meets her soon afterward riding through a wood in the company of her father, Sir Frederick Vernon, a political exile. Diana gives Frank a packet of papers that Rashleigh has been forced to give up; they are notes to the credit of Osbaldistone and Tresham. Frank’s father’s fortune is safe.

In the Jacobite revolt of 1715, Rashleigh becomes a turncoat and joins the forces of King George. At the beginning of the revolt, Sir Hildebrand had made his will, listing the order in which his sons are to inherit to his lands. Because Rashleigh has betrayed the Stuart cause, Sir Hildebrand substitutes Frank’s name for that of Rashleigh in the will. Sir Hildebrand is later captured by the royal forces and imprisoned at Newgate, where he dies. His four sons also die from various causes, and Frank inherits all the lands and properties that belonged to Sir Hildebrand. When Frank goes to Osbaldistone Hall to take over, Rashleigh shows up with a warrant for Diana and her father; but he is killed in a fight with Rob Roy. Frank becomes the lord of Osbaldistone Hall. At first, Frank’s father does not like the idea of his son’s marrying a Papist, but in the end he relents and gives his permission, and Frank and Diana are married.