The action takes place in the imaginary kingdom of Allemonde.
ACT I
During a hunt, Prince Golaud, grandson of the old king Arkel, gets lost in the forest. At a fountain he meets Mélisande, a weeping young woman who refuses to explain who she is. She also objects to Golaud retrieving a crown that has fallen into the water.
At the castle, Geneviève, Arkel's daughter-in-law, reads the king Golaud's letter to his half-brother Pelléas. The prince announces that he has married Mélisande, although he still knows nothing about her. Fearing Arkel's reaction, he asks Pelléas to send him a sign if the king agrees to accept his young wife. Arkel agrees, although he would have preferred his grandson to marry Princess Ursula after the death of his first wife. Pelléas appears. Marcellus, a dying friend, calls him to his bedside. But Arkel refuses to see him go, as Golaud is about to return and Pelléas' father is also in agony. Golaud returns to the castle in Melisande's company. During a walk, Geneviève and Mélisande find Pelléas near the sea, just as the boat that had taken the princely couple passes. The sea is calm but a storm is forecast. Geneviève leaves to inquire about little Yniold, born of Golaud's first marriage. Pelléas takes Mélisande home and tells her on the way that he might leave the next day, which affects her.
ACT II
In the park, Pelléas takes Mélisande to see a fountain that is reputed to restore sight to the blind. As the clock strikes noon, Mélisande loses her wedding ring in the water.
Golaud is injured after a fall from his horse at the twelfth stroke of noon. At his bedside, Mélisande confides her uneasiness to her husband. When he takes her hands to comfort her, he notices that the ring is missing. The young woman says she lost it in a cave by the sea. Golaud orders her to go and find it.
Mélisande goes to the cave accompanied by Pelléas. Pelléas advises her to observe the place carefully in order to be able to answer Golaud's questions. They notice three sleeping paupers and flee.
ACT III
Mélisande combs her hair at the window of a castle tower. Pelléas passes under the window and asks to see her hair untied. He tells her he is leaving the next day but Mélisande convinces him to stay. Her long hair falls down to Pelléas, who is ecstatic about it. Golaud surprises them and reprimands them.
The next day, Pelléas and Golaud enter the castle's underground passageway, from which the smell of death emanates. When they emerge at midday, Golaud asks his half-brother to avoid his wife, especially as Mélisande is pregnant.
In the evening, outside the castle, Golaud questions his son Yniold about the nature of the relationship between Pelléas and Mélisande. His jealousy makes him lose his mind, and he orders the boy to spy on the young woman.
ACT IV
Pelléas asks Mélisande to join him that evening near the fountain of the blind. His father having recovered, he prepares to leave.
Arkel tells Mélisande of the pain the atmosphere of death in the castle had caused him, and of the hope that this cure now provides. Golaud appears, mad with jealousy. He bullies Mélisande to the point that Arkel has to intervene.
In the garden, Yniold tries to recover a golden ball that has fallen under a rock. He sees some sheep passing by, which the shepherd prevents from going to the stable.
By the fountain, Pelléas waits for Mélisande, determined to bid her farewell. When she finally appears, Pelléas explains that he must leave because he loves her. Mélisande confesses her love in return and the two lovers kiss. Golaud surprises them. He strikes Pelléas with a fatal blow and goes after Mélisande.
ACT V
At Mélisande's bedside, Golaud is angry at having hurt his wife. The doctor tries to reassure him that the wound is not fatal. When Mélisande wakes up, Golaud asks to be left alone with her. He wants to know if she has been guilty in her love for Pelléas, which she denies. Golaud refuses to believe it and loses his temper. Arkel enters the room and presents Mélisande with the little girl she has given birth to in her sleep. As Golaud tries to talk to her again, the young woman dies. Arkel recommends that the newborn baby be cared for: she must live in her mother's place.