Marie is happy to tell this story that she loves. Is is derived from the Tristam and Isolde myth. Marie reminds us of the lovers, and that "their love was so pure that it caused them to suffer great distress and later brought about their death on the same day."
Tristam is in love with the wife of his uncle Mark, who is king. For this reason, the king banishes Tristam, who nevertheless is willing to run the "risk of death and destruction" to see his beloved again. He leaves his own land and travels to Cornwall, where the queen lives, traveling secretly and living with poor people to avoid detection. He learns through his hosts that the king has called his barons to court for revelry.
Tristam is excited at the news, since he can devise a plan by which to see the queen. He decides to hide in the woods near the path she will travel. While waiting, he cuts a hazel branch (covered in honeysuckle) in half, squares it, and writes his name on it, so that should the queen see it, she will recognize it as his and know he thinks of her and cannot live without her.
When the queen rides by with the group heading to court, she sees the wood and knows what it is. She asks her escorts to stop for a rest, and she moves a bit away from them with her faithful servant Brenguein. There she finds Tristam waiting, and they enjoy one another's company. She tells him how he might be reunited with the king, who felt bad about the banishment. When the time comes for them to part, they are quite saddened.
Because of his great happiness at the event, Tristam composes a lay using his skill as a harpist. It is this lay that Marie recounts.
This short lay relates an incident from the very well-known legend of Tristan and Isolde. Though scholars cannot place the exact date of Marie's compositions, it is likely that her audience would be extremely familiar with the legend from medieval romances, and so the meaning of the lay should be understood in terms of an audience who would know the story.
Of course, even if you don't, Marie makes certain that suspense is not at issue here: she tells us in the first section that the lovers' story will ultimately end with their tragic deaths.
Such tragedy is not Marie's interest here, however. Instead, she wishes to use the hazel branch and honeysuckle as a symbol of her favorite theme: the impossibility of private love in a public world. Marie tells us in the lay, "the two of them resembled the honeysuckle which clings to the hazel branch: when it has wound itself round and attached itself to the hazel, the two can survive together: but if anyone should then attempt to separate them, the hazel quickly dies, as does the honeysuckle." Their love is pure and they are intertwined to the core. In this way, the sign he prepares for the queen will only be discernible to her; it is a sign of their own private love.
Yet the lay makes clear that this happy moment of reconciliation is the anomaly in their relationship. Their time together is happy, but it is short and unlikely to be repeated regularly, if at all. The world requires them to go through such complications merely to talk, and they are separated by circumstance. Marie emphasizes this point by continually referring to her only as "the queen," even though the audience would know her as Isolde. Marie wishes to remind us that she is trapped by her public function, and this disconnect is what will ultimately lead to their tragic demise.
All of this is implicit in what is mostly a happy little lay, in which lovers come together. Lastly, this lay can be interpreted as a testament to the power of art and imagination; after all, it is something that Tristam creates that allows them to enjoy their feelings for a moment. Considering how heavily Marie stresses her skill and the importance of relating lays, it could be argued that she wishes to remind her audience that art and creation are what truly celebrates love, whereas normal life wants only to destroy it.