Lorna Doone is narrated by John Ridd, a yeoman of Oare parish, Somerset. The novel begins in 1673, when John is 12. His father has been murdered by the Doones of Bagworthy, a clan of outlaws who terrorise Exmoor. Born of noble stock, they are considered to be above law or justice, and act with impunity.
When John is 14, fate takes a dangerous turn. While out fishing, he ventures deep into the Bagworthy water. He loses his footing and is carried away by the river – into Doone Valley. Here he meets a young child, Lorna Doone, and narrowly avoids being discovered by the men of the Doone clan. Lorna shows John how to get safely out of the valley, and he escapes home, intending never to set foot near Doone Valley again.
He keeps his resolution until his 21st year. His uncle, Reuben Huckaback, comes to visit John's farm. He is attacked en route by the Doones, and treated brutally. Uncle Reuben insists that John take him to a viewpoint where he can see into Doone valley, in order that he may plot his revenge. As Reuben and John gaze down into the valley, John sees the tiny figure of Lorna in the field where he first met her, and realises that he is fated to love her.
John finds his way back into the valley, and begins his secret and extremely dangerous courtship of Lorna. She is 15, and not as open to falling in love as John, but he eventually wins her heart. The romance seems destined to end in tragedy. They can meet only rarely, and in secret. Lorna is certain that the Doones will never let her leave the valley alive. Carver Doone, the vilest member of the clan, has already executed one would-be-rescuer.
When Lorna’s grandfather, Sir Ensor Doone, head of the clan and Lorna’s protector, falls gravely ill, the danger facing Lorna increases dramatically. Ensor’s second in command, the Counsellor, plans to force Lorna to marry his son Carver as soon as Sir Ensor dies. John manages to spirit Lorna out of Doone valley and brings her home – putting the farm and his family at great risk of reprisal.
John, however, learns by chance that Lorna is not a Doone at all, but the daughter of Scottish nobility. The Doones kidnapped her as a very small child, and raised her as their own. This information results in Lorna being sent to London to live under guardianship, as she is now extremely wealthy and is not yet 18. John fears he will have to give her up, as a yeoman has no right to make claims on a noble lady. Lorna remains committed to John, however, and assures him that they will eventually be together.
The Doones continue to terrorise the countryside. After a particularly vicious attack by them on a local family, John leads a party of local farmers in an assault on Doone Valley. All the Doones are killed, with the exception of the Counsellor and Carver, who both escape.
Lorna and John are granted permission by the King to marry, and finally look set to attain their happy ending. As they are saying their vows, however, Carver appears and shoots Lorna. John rushes after Carver, and they fight hand to hand. John is deeply wounded, and Carver dies. John and Lorna are both on the verge of death, but they are slowly nursed back to health, and are finally reunited as husband and wife.