The story takes place in Britanny (sometimes referred to in the tale by its old Latin name of Armorica) and concerns the consequences of a rash promise. Dorigen is distressed, fearing that her absent husband Arviragus (or Arveragus) may be killed when he returns to Britanny as the result of his ship striking one of the many rocks that lie off the region's coast. When another man called Aurelius declares his love for Dorigen, she tells him that she will only agree to become his lover if he can remove every rock from the sea near Britanny, believing that to be impossible. However, Aurelius is able to procure the services of a magician who makes the rocks appear to vanish. When Arviragus fins out about his wife's promise, he urges her to keep her word.
In Britanny there lives a knight called Arviragus. After much difficulty, he is eventually able to win the love of the lady Dorigen. Arviragus promises Dorigen that he will never be suspicious of her. Dorigen, in return, tells her husband that he will never have any reason to be suspicious of her.
After they have been married for a year, Arviragus goes to England in order to win greater fame as a knight. He leaves Dorigen behind in their castle by the sea. Arviragus is absent for two years and it is a miserable time for Dorigen, in spite of her friends' constant efforts to distract and amuse her. Eventually, Dorigen receives a letter from her husband in which he says that he will come home soon. However, the letter only causes Dorigen more distress. While walking by the sea with her friends, she always comments on the many rocks that lie just off the coast. She is afraid that Arviragus will be killed if his ship strikes one of those rocks. Dorigen's friends are forced to take her to other places and to avoid walking by the sea.
One spring day, Dorigen is approached in a garden by a young man named Aurelius. Aurelius has been in love with Dorigen for years but had never spoken of his love until that day. Dorigen refuses to be unfaithful to her husband but promises Aurelius that she will change her mind and become his lover if he can remove every rock from the sea off the coast of Britanny. Both Dorigen and Aurelius consider the task to be impossible.
Aurelius suffers more than two years of being love sick for Dorigen, during which time Arviragus returns home and he and Dorigen live happily together once again. Only Aurelius' brother knows the true cause of his distress. Eventually, Aurelius' brother remembers that he knew of magicians in the French city of Orleans who were able to make all manner of objects appear or disappear. The two brothers set off for Orleans but a magician approaches them and says, "I know the cause of your coming", before they reach the city. The magician takes the two brothers to his house and proves his skill by showing them visions of a hunt, a tournament and a dance. He agrees to make all rocks disappear from the Breton coast but wants to be paid at least a thousand pounds for performing the task. Aurelius promises to pay the magician any price he names.
The magician returns to Britanny with Aurelius and his brother. After lengthy studies, he finds a time when the planets are in the right position to make the rocks of Britanny appear to vanish. Aurelius reminds Dorigen of her promise and shows her that the rocks have gone. Dorigen returns home, Arviragus is absent at that time, and for two days thinks about how she would rather commit suicide than be unfaithful to her husband. However, when Arviragus returns, although it greatly upsets him, he tells Dorigen that she must keep her word.
Dorigen and Aurelius meet each other on the way to the garden. Dorigen tells him that she is doing what her husband instructed her to do, in spite of the distress that it causes them both. Aurelius is moved by Dorigen's words and the love that she and Arviragus have for each other. He releases Dorigen from her promise.
Aurelius has difficulty raising the money which he owes the magician. He goes to see him to ask if he can pay his debt off in installments. The magician reminds Aurelius that he kept his side of the bargain and allowed the young man to make love to Dorigen. Aurelius tells the magician that he did not make love to Dorigen and explains why. The magician is impressed by the nobility that was shown by both Arviragus and Aurelius. To show that he can be just as noble himself, he cancels Aurelius' debt.
The Franklin ends his tale by asking his listeners which of the three characters, Arviragus, Aurelius or the magician, they think showed the most nobility of all.