Lost in the forest, Golaud discovers a young girl in tears. On seeing him, she takes fright and, despite all his questions, refuses to reveal her name, which is Mélisande. Golaud explains that he is a prince, the grandson of Arkel d’Allemonde, and takes her with him. Six months after his marriage to Mélisande, Golaud writes to Pelléas, his beloved half-brother, to inform him of the change in his situation. He asks him to intercede with Arkel on his behalf and to light a lantern visible from the sea if the king agrees to receive his wife.

The lantern will tell him if he can return home or must carry on his way. A disquieting, stormy atmosphere hangs threateningly over the sea. Mélisande, Pelléas and Geneviève, the mother of the two brothers, watch as the boat which has brought the young couple to Allemonde sails off to an unknown future. Pelléas leads Mélisande to a fountain in the garden of the castle. She plays with her wedding ring above the water and, as the clock in the tower sounds noon, drops it into the basin. At that very moment Golaud falls from his horse.

That evening, Mélisande nurses her husband, who notices the absence of the ring from her finger. She tells him she has lost it collecting shells in the cave on the beach. Golaud immediately orders her to go and look for it with Pelléas. Although they are perfectly aware that the ring is not to be found there, Pelléas and Mélisande go to the cave so as to be able to describe the place if the need arises. But when the moonlight reveals some sleeping beggars, they turn back frightened. Mélisande sings at her window.

From the garden, Pelléas persuades her to lean out towards him. Her hair falls to the ground and envelops Pelléas. He only disentangles himself when Golaud approaches. Golaud drags Pelléas down to a water tank in the subterranean vaults of the castle and forces him to lean out over the abyss. On a terrace on the way out, he asks his half-brother never to see Mélisande again. Golaud speaks to the young Yniold, his son from a first marriage, seeking to obtain information about the relationship between Pélleas and Mélisande. As the child does not know enough, he lifts him onto his shoulders so that he can see into Mélisande’s bedchamber and tell him what she is doing with Pelléas, who is there.  

Pelléas must leave. He arranges a final meeting with Mélisande near the fountain in the castle grounds to bid her farewell for ever. Arkel is trying to raise Mélisande’s spirits when Golaud arrives. Irritated, he ill-treats his frightened wife until the king makes him see reason. Yniold is looking for his golden ball. In the distance he can see a flock of sheep being taken for slaughter. In their final leave-taking, Pelléas and Mélisande avow their love for each other. Golaud suddenly appears and kills Pelléas.

Mélisande flees, pursued by her husband. Golaud, who has grievously wounded Mélisande, watches over her bed with Arkel and a doctor. When the young woman regains consciousness, he desperately seeks to discover whether she has betrayed him with Pelléas. Mélisande dies, however, without having revealed her secret.