King Conchubor keeps Deirdre, the beautiful young woman he resolves to make his bride, at the home of Lavarcham, the old nurse, on Slieve Fuadh. One rainy evening, Conchubor and his friend Fergus arrive to find that Deirdre, to the king’s displeasure, is still out gathering nuts and sticks in the woods. Lavarcham warns the king that Deirdre will not be anxious to see him, and she repeats the old prophecy that Deirdre was born to bring destruction into the world. When Deirdre comes in, the king presents her with rings and jewels and remonstrates with her for staying out in the woods. Deirdre defends her behavior and says that she has no desire to go to Emain to become queen.

Conchubor pleads with her, talking of his loneliness, his love for her, and the rooms he prepared for her in his castle at Emain. Deirdre insists that in spite of the fact that she is pledged to Conchubor she would prefer to remain in the simple cottage with Lavarcham as long as possible. Conchubor, growing impatient, insists that she be ready to go to Emain and become his queen within a few days.

After he leaves, Lavarcham urges Deirdre to be sensible and bend to Conchubor’s wishes, but Deirdre keeps talking about other defiant legendary heroines and about Naisi and his brothers, the bravest men in the woods. Deirdre goes to dress elegantly for one of the last nights of her freedom.

In the meantime Naisi and his brothers arrive at the cottage to take refuge from the storm. Lavarcham is not eager to let them in, but they claim that a beautiful lady whom they met in the woods promised them refuge from the storm. They enter, but Lavarcham, sensing trouble, tries unsuccessfully to get rid of them. They are still in the room when Deirdre returns. Deirdre provides food for Ainnle and Ardan. When they leave the cottage she asks Lavarcham to leave also. Alone with Naisi, she tells him of Conchubor’s imminent suit. Deeply in love by this time, they decide to marry and run away in spite of their knowledge of the troubles foretold. They ask Ainnle, who returns to the cottage, to marry them before they flee into the night.

Seven years pass during which Deirdre and Naisi, with Ainnle and Ardan, live happily beside the sea in Alban. One day Lavarcham arrives to announce that Fergus is on his way with peace offerings from King Conchubor and to plead with Deirdre to accept the king’s offer. Deirdre insists on her loyalty to Naisi. Owen, Conchubor’s trusted man, arrives with word that Naisi and Fergus are already talking on the path below; he rudely advises Deirdre to leave Naisi and return to the king. Owen thinks that seven years of love are more than enough and that Deirdre will one day be old and yearn for the comfort of the royal palace. Owen also reveals that he is jealous of Naisi and hates him because he killed Owen’s father some time before.

Fergus, on his arrival, says that Conchubor in his peace offering invites Naisi and Deirdre back to Emain in peace. Naisi and Deirdre wonder if they should accept the offer. They talk of age, the possible death of love, and the happiness of their seven years, despite some difficult times, at Alban. They experienced such perfect years, they decide to accept Conchubor’s offer and return to Emain; they feel they will never know such complete happiness at Alban again. Owen returns, screaming that it is all a plot, and then runs out and splits his head against a stone. Believing Owen mad, Naisi and Deirdre accept Fergus’s promise that no trick is involved, and they set out for Emain to meet Conchubor again.

Lavarcham, arriving first to speak with Conchubor, finds him a lonely old man. After assuring the king that he can never gain Deirdre’s love, she reports that Owen, despairing of ever gaining Deirdre, ran mad and destroyed himself. Conchubor’s warriors arrive and report that they separated Naisi and Deirdre from Naisi’s brothers. When Naisi and Deirdre arrive, they find themselves in a tent. A freshly dug grave is concealed by curtains next to the tent. They speak mournfully, for they strongly suspect a plot against them. Conchubor returns, welcomes them, and seems, in spite of the evidence of the tent, the grave, and warriors lurking nearby, to mean his offer of peace seriously. Then, as he and Naisi are about to clasp hands of friendship, Naisi hears his brothers cry for help. Naisi starts to leave, although Deirdre pleads with him to stay. Naisi curses the softness of women and runs out. The king’s warriors kill Naisi as they killed his brothers.

Conchubor urges Deirdre to end her mourning for Naisi and become his queen. Deirdre continues to lament and will have nothing to do with Conchubor. Fergus appears and announces that he burned Emain because the king went back on his pledge not to harm Naisi. Fergus, who acted in good faith, tries to protect Deirdre, but Deirdre uses Naisi’s knife to commit suicide and join him in another world without defiling their love. After Deirdre’s death, all mourn. Conchubor, old and broken, is led away by Lavarcham.