Marie introduces "Laüstic" as a famous lay known in "Rossignol" in French and "Nightingale" in English.
There once were in St. Malo two knights who lived nearby one another. Both have good reputations, with the first having a beautiful, courtly wife, and the other being known for prowess and adventure. The young man is much in love with his neighbor's wife and thanks to both his persistence and reputation, she gives in to his entreaties and loves him as well. Marie adds that she loved him "because he lived close by."
They are never able to meet because of her husband's diligence, but their proximity allows them to speak from their respective windows over the courtyard, and to toss gifts to one another.
One summer, the lovers begin the tradition of rising at night, wearing their mantles, so they can adore one another through the windows. The lady's husband grows angry at her constant exits from bed, and she tells him she gets up to listen to the glorious nightingale's song. The husband, now revealed to be spiteful, then plans to ensnare and kill the nightingale.
His servants rig the entire courtyard with trap snares until the bird is captured and given alive to the knight. He brings the bird to his wife and tells her she will now be able to sleep because he has caught the bird. When she asks for the bird, he kills it and throws it at her, staining her dress with its blood.
The lady knows she can no longer rise to look at her beloved, and is worried he will not understand her absences. She entrusts a servant with the bird and asks him to bring it to the neighbor. The servant complies, and the knight understands and is saddened to see the bird's corpse. He has the bird put in a small coffin, and carries it around with him from that day onwards.
"Laüstic," one of the most famous of the lays, uses several common fairy tale archetypes, including some (like the lady imprisoned by a spiteful husband) that are echoed in other lays. The poem uses these archetypes to create a moving story in an impressively short amount of lines.
This lay seems to have produced the most disagreement amongst scholars. Where some see the nightingale as a symbol of love, others see it as a symbol of doom. The latter interpretation seems more in line with the rest of Marie's work, which tends to stress the impossibility of private love in the public world. Where the woman and her neighbor maintain a chaste adoration of one another, the world (represented by her husband and his servants who help him ensnare the bird) is desperate to tear such a private love down.
And yet the love in this poem can be viewed as more suspect than that of some other lays. For one, it is never consummated, which doesn’t make it untrue, but does imply that the relationship was always conducted at a distance. That is, the lovers are to an extent emboldened by their fantasy of one another, by the glimpses they get. In some ways, the song of the nightingale – which one could go so far as to say represents poetry or song, and by extension those stories Marie tells – then represents the power of imagination and fantasy. In this interpretation, the nightingale that the knight carries around is a reminder of the power we have to create fantasy, and also of how those fantasies cannot survive the demands of a cruel world.
One can also view the poem through the lens of a criticism of chivalry, which Marie has certainly conducted in other lays. The young knight is described in terms of then-contemporary chivalrous values, known for "prowess" (with ladies, too?) and his adventures. Considering this, could his neighbor's wife be yet another adventure? That doesn't mean he doesn't actually love her – indeed, the idea of chivalry involves commitment to the ideal adventure, even if the adventure is being pursued as its own end. In this interpretation, the knight is engaging in a selfish love, and perhaps so is the lady, who sees in her fantasy of him an escape from the spiteful husband.
No matter how you choose to interpret the nightingale, this poem is extreme in its message. Whatever beauty or tragedy is suggested by the bird ends up as a bloodstain on the lady's dress, for the world always comes crashing in.