Geneviève reads a letter from her son, the widowed Golaud. In it he describes how he first encountered a mysterious young woman—who cannot give a clear accounting of herself or her origins—weeping by a fountain. It is six months after this meeting, the letter continues, and though Golaud has married the enigmatic Mélisande against his grandfather’s wishes, Golaud is eager for his grandfather, Arkel, to receive them. Arkel consents.
In the garden of Arkel’s castle, Mélisande meets Golaud’s half-brother, Pelléas, who takes her to a fabled well. As Mélisande plays with her wedding band, it disappears deep into the water.
Golaud awakes from a prophetic nightmare; Mélisande comforts him and reveals her unhappiness about being in the castle. Golaud notices that her ring is missing. He implores Mélisande to search for it immediately and suggests that she enlist Pelléas to assist with the futile task. Mélisande combs her long hair in a castle window as Pelléas passes by. She leans out to cover Pelléas in her cascading tresses. Golaud surprises them in what he considers their child’s play. Golaud warns Pelléas to avoid Mélisande.
Increasingly jealous, Golaud questions his young son Yniold about what he has observed of Pelléas and Mélisande’s relationship. Arkel, alone with Mélisande, informs her of Pelléas’s father’s improving health and expresses concern for her unhappiness. Golaud enters abruptly and announces Pelléas’s imminent departure. Golaud grabs Mélisande by her hair and drags her across the floor.
As they say their good-byes, Pelléas and Mélisande profess their love. Golaud interrupts them and kills his brother. Immediately remorseful, Golaud nonetheless demands the truth of Mélisande as to whether she loved Pelléas. Mélisande dies without answering to Golaud’s satisfaction.