Euryanthe is betrothed to Count Adolar. In a hall in the palace of King Louis of France the count sings the praises of his promised bride. Lysiart, Count of Forest and Beaujolais, challenges the fidelity of the maiden and asserts that he can win her should he care to try. Adolar stakes his lands and fortune on the faithfulness of Euryanthe and demands that his friend shall show some proof of his victory should he win one. In her castle at Nevers, Euryanthe has given refuge to Eglantine de Puiset, the daughter of a mutineer. This woman is enamoured of Adolar and, under the pretense of friendship for her benefactor, she secretly determines to effect Euryanthe's downfall and rupture her attachment to Adolar. Herein she is assisted by Lysiart, who has unsuccessfully attempted to gain the favor of Euryanthe. To Eglantine, Euryanthe has confided a secret given her by Adolar. The latter's sister Emma had lost her lover in battle, and had killed herself by drinking poison from a ring. Her soul could find no rest until the ring, lying in her tomb, should be moistened with the tears of an injured and innocent maiden. Eglantine visits the tomb, abstracts the ring, and gives it to Lysiart. The latter then displays the jewel to Count Adolar, who is convinced that his betrothed is unfaithful, since she must have betrayed the secret known to him and her alone. He takes Euryanthe into the desert, intending to kill her, but they are attacked by a serpent and the girl throws herself between the reptile and her lover. Adolar kills the serpent, but cannot find the heart to murder she who would have given her life for his. He leaves her to her fate. Euryanthe is found by the king and his hunters, and to the monarch she unfolds the story of her woe and the treachery of Eglantine. Meanwhile, Eglantine has become affianced to Lysiart, and the wedding is about to be celebrated in the Castle of Nevers when the woman is suddenly stricken with hysterical remorse. She thinks that Euryanthe appears to her as a ghost and, in her ravings, divulges the plot. Lysiart, in fury, slays his bride, but is at once seized by order of the king, who enters with Adolar as Eglantine breathes her last. Explanations follow, and Euryanthe, who makes her appearance at the last moment is once more taken to the heart of her beloved.