John Ridd is engaged in a schoolboy fight in the yard of Blundell’s school when John Fry, employed by Ridd’s father, comes to take the boy home. Before the two leave, however, young John completes his fight by knocking out his opponent. On their way home through the moorlands, the man and boy are nearly captured by members of the outlaw Doone band, which has been ravaging the countryside by stealing and killing. When John Ridd reaches his father’s farm, he learns that the Doones attacked and murdered his father only a few days previously. This incident stimulates the desire for revenge in all the residents of the parish of Oare, for the murdered man had been greatly respected.

John settles down to the responsibilities that the death of his father have thrust upon him. At first, his time is greatly consumed by farm work as he grows into the largest and strongest man in the Exmoor country. As he matures, John learns much about the wild Doone clan. There is one Doone, however, toward whom he feels no animosity: Lorna Doone, the beautiful daughter of the man supposed to be the murderer of John’s father. On first sight of the young woman, John had been stirred by her beauty. Ever since, he has been in great conflict, as he understands that his passion is directed toward someone he ought to hate for his father’s sake. After John’s great-uncle, Master Reuben Huckaback, is attacked and robbed by the Doones, he goes with John to swear out a warrant for their arrest, but he has no luck because the magistrates are unwilling to incur the enmity of the Doones.

Over time, John is drawn deeper into a relationship with Lorna Doone. The two meet secretly in Doone Valley, and she tells him the story of her life with the outlaws. She has always loved her grandfather, Sir Ensor Doone, but she fears and has come to hate the rough, savage sons, nephews, and grandsons of Sir Ensor. This hatred is increased when Carver Doone cold-bloodedly murders Lord Alan Brandir, a distant relative who had come to take Lorna away from the Doones.

About this time, John is called to London to serve the cause of King James II’s tottering throne. There he discloses all he knows of the Doones’ activities and of the false magistrates who seemed to be in league with them. He is warned that Tom Faggus, a highwayman who is John’s cousin, might go to the gallows soon. Because John refuses to accept bribes or to become the dupe of sly lawyers in the city, when he eventually returns to his mother and his farm he is not a penny richer or poorer than when he left.

In the meantime, John’s concern over Lorna, who has two suitors among the Doones themselves, has almost unhinged his mind. He is delighted to discover that Lorna, still only seventeen years old, has refused both her suitors. At the same time, he fears more than ever that he will lose his chance of winning Lorna, the ward of the outlaws he has pledged to help the king destroy. He at last, however, wins Lorna’s agreement, and, with her support, he feels that nothing can stop him.

At home, the love of John’s sister Annie for her cousin, Tom Faggus, reminds John of his duties as his father’s son and plunges him into worries about his mother, Annie, and the farm. John’s mother had other plans for his marriage, but when he reveals to her the only course his love must take, he succeeds in changing her mind. In the meantime, Master Jeremy Stickles has brought news of the rising of the duke of Monmouth and of troubles brewing for the king.

Suddenly, the signals that Lorna has been sending to John stop. John takes the great risk of descending into the Doone hideout, where he discovers that Lorna has been kept in her rooms because she has refused to marry Carver Doone. John manages to talk to her, and she pledges never to give in to her family. As he leaves, he narrowly escapes capture and, at the same time, overhears the outlaws’ plot to kill Jeremy Stickles, the king’s messenger, and thus is able to save Jeremy’s life. The Doones’ plot to kill Stickles brings further plans for retaliation from the king’s men.

Old Sir Ensor Doone is close to death. Before he dies, he gives the union of John Ridd and Lorna Doone his blessing, and he presents to Lorna the glass necklace he has kept for her since she was a child. John then takes Lorna home with him to his mother’s farm. Jeremy Stickles goes south to muster forces for the destruction of the Doone clan, and the counselor of the Doones takes advantage of Jeremy’s absence to visit the Ridd farm in order to make a truce with John. His offer is rejected, but he throws trouble into the lovers’ path by telling them that Lorna’s father murdered John’s father and that his own father was the murderer of Lorna’s father. He also tricks them into giving him Lorna’s necklace, which by now, through the word of Tom Faggus, they know to be made of diamonds.

Uncle Reuben Huckaback grows interested in having John marry his granddaughter Ruth, and he takes John to see the gold mine he has just bought. Upon returning from the mine, John learns that Lorna has disappeared. She has been taken away by the Dugals, who claim her as their missing heiress.

When Tom Faggus joins the rebels against the king, John, at his sister Annie’s request, goes to find him and bring him back to safety. When John finds Tom, he is almost dead. John is then taken prisoner and is nearly executed; he is saved only by the arrival of his friend Jeremy Stickles.

John travels to London, where he sees Lorna. By good chance and by virtue of his great strength, John overcomes two villains who are attempting to rob and kill a nobleman; afterward, he learns that the man is Lorna’s relative. In return for John’s brave deed in saving the nobleman, the king gives John the title of knight and also has the court of heralds design a coat of arms for John’s family.

When John returns from London, covered with honors, he discovers that the Doones have been raiding once more. John then finally achieves his long-awaited revenge. The Doones are routed, their houses are burned, and their stolen booty is divided among the local citizens who put in claims for redress. Lorna’s necklace is also recovered. In addition, the Doones’ counselor reveals that it was Carver Doone who killed John’s father.

Arrangements are at last made for the wedding of John and Lorna. At the end of the ceremony in the church, Carver Doone, out of great jealousy, shoots Lorna. Without a weapon in his hand, John rushes out in pursuit of Carver; when he finds him, the greatest battle between two men ever told of in books takes place. It is a fight of giants. As John feels his ribs cracking in Carver’s tremendous hug, he fastens his own iron grip on his enemy’s arm and rips it loose. Then he throws his crushed and bleeding enemy into a bog and watches as Carver Doone is sucked down into its black depths. The greatest enemy of John Ridd is at last destroyed. John returns to his bride and learns that she might survive her wound; she does, and John and Lorna live in peace and plenty among their friends to a hearty old age.