Euryanthe is engaged to Count Adolar. In the palace of King Louis of France, Adolar sings about his future wife. Lysiart, Count of Forest and Beaujolais, challenges Euryanthe's fidelity and states that he can win her. Adolar is sure of his fiancé's faithfulness and demands some proof of his victory if he ever wins. In her castle at Nevers, Euryanthe has given refuge to Eglantine de Puiset, the daughter of a mutineer. This woman is in love with Adolar and, pretending to be friend to Euryanthe, she secretly plans to ruin her relationship with the Count. She is assisted by Lysiart, who has unsuccessfully attempted to gain the favour of Euryanthe. Euryanthe has confided to Eglantine a secret given her by Adolar. Adolar's sister Emma had in fact lost her lover in battle and had killed herself by drinking poison from a ring. Her soul can't find rest until the ring, lying in her tomb, gets moistened with the tears of an injured and innocent maiden.
Eglantine visits the tomb of Adolar's sister, gets the ring and gives it to Lysiart who shows it to Adolar, who is now convinced of his fiancé's unfaithfulness, since she has betrayed the secret which was known only to him and her. Despite the girl's demonstration of innocence, he takes her into the desert.
Adolar has brought Euryanthe to a rocky gorge, intending to kill her, but they are attacked by a snake. The girl throws herself between the reptile and her lover, Adolar kills the snake, but cannot make himself to murder her, so he leaves her to her fate. Euryanthe is found by the king and his hunters, whom she tells the story of her woe and Eglantine's betrayal. Meanwhile, Eglantine has become engaged to Lysiart, and the wedding is about to be celebrated in the Castle of Nevers when the woman is stricken with remorse. When she sees Adolar wearing a black widow vest, she thinks that is Euryanthe appearing to her as a ghost and, believing her dead, reveals her plot. Lysiart, in fury, kills his bride to be. The king arrives and explanations follow; Euryanthe, who makes her appearance is once more taken to the heart of her beloved.