Marie begins "Yonec" by restating her purpose, to tell of the adventures that she knows. She professes a particular intention to tell about Yonec, about how he was born, and about how his father (Muldumarec) first met his mother.
Marie tells of a rich man who once owned a great amount of land in Caerwent in Brittany, and was thus "acknowledged lord of the land." Because he is very old and has such an inheritance, he takes a beautiful wife to produce an heir. However, because of her great beauty and character, he locks her away and has his old, widowed sister act as guard for the girl. He is afraid to lose her.
He keeps her there for seven years, and though they have no children, persists in his overprotectiveness. During those years, she laments and cries so terribly and often that she loses her beauty and wishes for death.
In April, as spring approaches, the lord one day leaves to hunt, and the old woman leaves the girl alone and locked in. She gives a long lament, cursing the man's foolishness that prevents her both from happiness and from hearing mass, cursing her parents who gave her to him as wife, and cursing the fates which prevent her from having a knight to rescue her as she heard tell of in old stories.
At that moment, she notices a stately hawk in her window. It flies in and watches her a while before it turns into a handsome knight, who confesses he has loved her for a long time but could not visit her until she wished for him. Her lament about being rescued by a knight has called him to her. She is quickly smitten but resists until he promises he believes in God. He gives a lengthy defense of his faith, and then proposes a plan to prove it to her: he will assume her shape and pretend he is ill so that the chaplain will come to speak a service over him and he can take communion. She agrees, and he lies next to her, but does not make any move.
When the old woman checks in, the knight has taken the lady's shape and pretends to be sick. Though the old lady insists no visitors are allowed, the illness is so well feigned that a chaplain is brought to give communion and speak prayers over her.
With the lady's worries thus assuaged, she and the knight enjoy a nice time in bed, after which the knight takes his leave. Before he goes, he tells the lady he will visit whenever she pleases, but warns her to "observe moderation" so that they are not discovered by the old woman, whose betrayal would most likely lead to his death.
The woman reclaims her beauty and happiness, as she is now quite content to be locked in the tower where her lover visits her as soon as her husband leaves. However, her husband, noticing her marked improvement, questions his sister about the change, and the old woman agrees that the lady is now more willing to be alone. The lord proposes that the old lady hide herself to discover what is the cause of such a change.
Three days later, the lord pretends to leave to visit the king, and his sister hides behind a curtain in the room, from which vantage she observes not only the rendezvous with the knight, but also his transformation from hawk to man. She betrays them to the lord, who makes traps from sharp razors that he fits into the window to slay the hawk on its next visit.
That visit comes the next day upon the lord's exit to hunt. The lady calls her knight, but when he enters, he is ripped fatally by the razors. He bleeds on her bed and laments, "I told you what would come of it: your appearance would slay us." When she faints from sorrow, he comforts her, telling her that she is pregnant with their child, a boy who will grow to be valiant, and who will avenge their loss by killing the old husband. The boy will be called Yonec.
His suffering is great, so he leaves. Distraught, she flees after him, leaping "a good twenty feet" from the window, undaunted by her nakedness. She follows the trail of blood he left behind up to an opening in a hill, through which she passes in complete dark until emerging in the light of a meadow. There is a city nearby, and she follows the blood into the city, all the way to the castle at its center, all the while encountering nobody. She follows the blood through two chambers in which knights sleep, until she finally finds her beloved in his bedroom at the center of the palace. Marie pays particular attention to the splendor of his bed, which is made of gold more impressive than any on Earth.
She swoons before him, but he comforts her again, all the while warning her to flee quickly since his own people will hold her accountable for his death. She is afraid that her husband would kill her, so he gives her a ring that will cause him to forget all he has seen, as well as a sword that is meant to be given to their son Yonec to slay the old man. The knight tells her how it will happen: many years from then, they will go as a family to a feast, and pass an abbey at which they will hear the story of the knight's death. The woman is then to give the sword to Yonec and tell the boy the truth, and they will see how he acts.
Comforted, she leaves her beloved, wearing a tunic he gave her and holding close the two other gifts. As she flees back across the meadow, she hears the bells ringing in mourning in the town and is saddened. She makes it back home, where her husband shows no sign of remembering what happened.
They raise her son Yonec well, and he is much loved for his valor. Once he is older and has become a knight, the lord brings his family one year to the feast of St. Aaron. They are housed at an abbey, and after mass intend to leave when the abbot asks them if he can show them around. Amongst his many rooms is a tomb richly decorated, and when the family inquires who is buried there, many of them cry in mourning, saying it was the best knight they had ever known, the king of that land, and they wait each day for his son to arrive.
The lady realizes what is afoot and she gives to Yonec the sword, telling him the story. She then faints over the tomb, dead. Yonec, realizing what has happened, beheads his stepfather. The people of the land bury the lady there in honor and make Yonec their lord.
Marie ends by stressing that this is a sad tale, and that the lay was composed "about the sorrow and grief that they suffered for love."
Though this lay is called "Yonec," Marie's short epilogue makes clear its true subjects are the parents of the titular character. In this vein, the section of the lay after Yonec's birth is significantly shorter than that detailing the meeting of the parents, suggesting that the true meaning and significance lies in this former section.
The three usual themes are at work here: the danger of selfish love, the impossibility of private love coexisting in the public world, and the importance of moderation.
In terms of selfish love, the old man is emblematic of it. He keeps his wife locked up in the tower, oblivious to how this robs her of her beauty and seemingly oblivious to her unhappiness as a potential cause for their lack of an heir. Compared to her, who wants most of all the chance to hear mass and give thanks to God, his love is selfish and stifling, and sets the stage for what is to come.
Luckily, the woman is granted a reprieve from unhappiness from another world. As in "Lanval," Marie's description of both the knight and his world are given particular attention, as though to stress to the audience that they are not of this world. If it is not obvious that the knight is supernatural from his ability to transform to hawk, then the world Marie encounters as she follows his wounded body to his city should be adequate proof. It is a classical and fairy tale archetype, this descent through darkness and into light that the woman takes, and the fact that she follows him to the center of that world, all the way to his bedroom at the center of the castle, stresses that theirs is a private love that has no place in the ugly world of men.
However, Marie is constantly a pragmatist in this regard. Her recurring message is that such a private love – which only two lovers know with one another – can never maintain its sacredness in the world of men. Indeed, despite their discretion, her appearance is what gives them away. Note that the knight blames this – her appearance – and not her indiscretion or her husband for his demise. The point is that their sacred love will ultimately be quashed by the world. Were the story to follow a more 'realist' trend from this point (which means Marie would have no magical world into which to follow him), then all would be lost, their love negated.
Of course, what can easily be identified as equal cause of their discovery is the lady's lack of moderation. When the knight first gives her instructions for calling him, he stresses she must use moderation so as not to encourage too much dangerous attention. Nevertheless, her immense joy leads her to call him "night and day" and as a result her husband's suspicions are quickly ignited.
As in some of the other lays, magic plays a role in this lay, though it is as usual not the defining factor. Instead, Marie always leaves the ending up to human agency. As mentioned above, it is Marie's lack of moderation that causes the problems – the knight's magical ability only sets the stage. The story is suffused with magic, though. We have the magic world in which the knight lives, as well as the magic items he gives and his foretelling of the future. However, notice that even in the knight's foretelling of the future, he does not share how the story ends. He tells her she will give Yonec the sword and then "they would see what he would do." It is keeping with Marie's worldview that Yonec still has free will to act heroically or cowardly. (It should come as no surprise to the reader by now that he chooses the former, of course.)
One element quite pronounced in this lay is that of Christianity. It is implicit throughout all the lays, as one would expect given the historical background, but here it gets special attention. Not only does the woman lament above all her inability to hear mass when trapped, but the knight's speech defending his belief in God is quite lengthy, given Marie's usual economy. A simple "Yes, I believe in God," would have accomplished the same purpose. One wonders whether this was intended to speak to a particular patron for whom she intended the lay, or whether she means to link selfless love (which the lovers share) with the love of God, thereby contrasting it with that of the old man. After all, the final vindication occurs at an abbey, as though to suggest that love of God will lead to ultimate victory even in the midst of sadness. Many patterns speak to Christian stories as well – such as the woman's descent into darkness before coming up to light, and the way that the inhabitants of the abbey await the coming of a son who will carry on the work started by the father.