Golaud, lost in the forest while hunting, finds a young girl weeping by the side of a pool. There is a golden crown in the water but she vehemently refuses his offer to get it out because it had been given to her by someone who had frightened her and from whom she had run away. All she can tell him is that her name is Mélisande and that she comes from far away. With some difficulty he persuades her to go with him, as long as he doesn't touch her. 

Geneviève reads to the half-blind Arkel a letter written to Pelléas by his half-brother Golaud, in which he relates the circumstances of his meeting with Mélisande six months ago. He has now married her but knows no more of her story than he did then. He fears that Arkel will not accept this marriage but Arkel, though he had hoped to mend old feuds by marrying Golaud, a widower, to another princess, resigns himself to the inevitable. 

Pelléas appears, in tears at a letter from his friend Marcellus begging him to attend his deathbed, but Arkel reminds him that his father, gravely ill upstairs, has a greater claim on him. Geneviève reminds him to light the lamp which will signal to Golaud that he may return in peace. 

Mélisande comments on the darkness caused by the thick forests around the castle and Geneviève tells her that she was similarly struck when she first arrived there. Pelléas joins them and they watch a ship sailing past. Pelléas tells Mélisande that he is leaving. 

Pelléas takes Mélisande to an old fountain where she plays near the deep water, causing him to fear for her safety. She plays with her wedding ring and it falls in the water as the clock strikes mid-day. Pelléas advises her to tell Golaud the truth. 

Golaud is in bed, having been thrown from his horse on the stroke of mid-day, and Mélisande tends him. She tells him she is not happy in the castle but is unable to explain why. She says she believes that Pelléas does not like her, but this is not the reason; but she does admit to being oppressed by the gloom of the forest-surrounded castle. 

Golaud takes her hand and discovers the loss of the ring. She tells him it must have fallen from her finger when she was gathering shells in a cave near the sea. He commands her to go and look for it at once, even though it is night, for fear it might get washed away by the tide. She is to take Pelléas with her. 

Pelléas and Mélisande have gone to the cave, as he tells her she must be able to describe the place where she says she has lost the ring. Mélisande is frightened at the sight of three old paupers sleeping in the cave. 

Pelléas appears below Mélisande's tower room to tell her that he is leaving. At his request she stretches out her hand for him to kiss. He cannot reach it but Mélisande's long hair cascades over him and he embraces it and pretends that he will hold her there by it. Golaud comes upon them and upbraids them for childishness. 

Golaud takes Pelléas to a stagnant pool in the underground chambers of the castle. Pelléas feels as if he is suffocating and they leave. 

Pelléas rejoices in the fresh air and Golaud warns him to keep away from Mélisande. She is not to be upset because she is about to have a child. 

Outside Mélisande's window, Golaud tries to get his son Yniold to tell him what happens between Pelléas and Mélisande when he is not there. Yniold can only tell him that though they are always together he has only seen them kiss once. Golaud lifts the child to look into the room. He sees Pelléas enter but he and Mélisande only look at the light and say nothing. Yniold is frightened by his father's ill-suppressed violent jealousy and asks to be put down. 

Pelléas tells Mélisande his father has recovered and advises him to travel. He intends to leave at once but arranges to meet her by the fountain to bid her farewell. Arkel tells Mélisande he has felt sorry for her as the castle has been so gloomy. He hopes that her youth and beauty will open the way to a new era. 

Golaud bursts in with blood on his head, which he says comes from walking through a thorny hedge. He refuses to let Mélisande touch him and orders her to bring his sword. She is trembling and he assures her he has no intention of killing her, but bursts into a mad rage and drags her round the room by her hair. 

Yniold tries in vain to extract his golden ball from beneath a stone. A shepherd passes with his sheep. 

Pelléas waits for Mélisande. He tells her that he must leave because he loves her and she confesses that she has loved him since first seeing him. They embrace passionately, half fearfully, half defiantly, realising that the gates have been shut behind them and that Golaud is waiting in the shadows. Golaud kills Pelléas and Mélisande runs away. 

Although only slightly wounded by Golaud, Mélisande, who has given birth to a daughter, is dying. Golaud, feeling the essential innocence of the lovers, is filled with remorse, but this does not stop him from trying to find out whether their love was guilty. Mélisande denies it but he is unable to believe her and realises there will be no resolution to his torment. Mélisande's child is brought to her. She is too weak to hold her and feels pity for her. She dies quietly and Arkel says that it is now the child's turn.