Euryanthe is betrothed to Count Adolar. In a hall of the palace of King Louis of France in Prémery, 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. Eglantine is enamoured of Adolar, and under the pretence of friendship for her benefactor, she secretly determines to effect Euryanthe's downfall and rupture her attachment to Adolar. Lysiart, who has unsuccessfully attempted to gain the favor of Euryanthe, assists Eglantine. After questioning by Eglantine, Euryanthe confides a secret given to her by Adolar to Eglantine. The latter's sister Emma had lost her lover in battle, and had killed herself by drinking poison from a ring (the 'ghost' music from the overture is heard). Her soul can find no rest until the ring, lying in her tomb, should be moistened with the tears of an injured and innocent maiden. Euryanthe, who has been praying each night at Emma's tomb, had promised Adolar to keep this secret, and, too late, she repents having told it to Eglantine. After Euryanthe leaves, Eglantine sings how she will denounce Euryanthe to Adolar; Lysiart arrives in order to take Euryanthe to the palace.
At night, Lysiart sings both of his guilt and his love. Eglantine visits the tomb, abstracts the ring, and gives it to Lysiart, who had almost given up on his wager with Adolar. She lets him know the secret behind the ring, and he proposes marriage with Eglantine.
Before an assembly in the hall at Prémery, Adolar reveals his anxiety while still longing for his bride, who then arrives. Lysiart displays the jewel to Count Adolar, claiming that Euryanthe had told him about it. Adolar is convinced that his betrothed is unfaithful, since she must have betrayed the secret known to him and her alone. Euryanthe protests her innocence, Adolar gives up his possessions to Lysiart, and rushes off into the forest with Euryanthe.
In a rocky gorge, Adolar intends to kill Euryanthe, still protesting her innocence, and then himself. They are suddenly attacked by a serpent and the girl throws herself between her lover and the monster; Adolar kills the serpent. He cannot find the heart to kill the one who would have given her life for his, and he goes off, leaving her to her fate. Euryanthe longs for death, but the king and his hunters arrive on the scene, and she recounts the story of her woe and the treachery of Eglantine. Although joyful that she might see Adolar again, she collapses as they lead her away.
Meanwhile, Eglantine has become engaged to Lysiart, and the wedding is about to take place in the Castle of Nevers, when she is stricken with remorse. Adolar has entered in black armour with his visor down. Eglantine, struck by the silence of the courtiers, and still in love with Adolar, thinks that Euryanthe appears to her as a ghost. Adolar shows who he is, and challenges Lysiart to fight. The king appears, and to punish Adolar for his distrust of Euryanthe, tells him that she is dead. Eglantine, triumphant at the supposed death of her rival, makes known the plot and is slain by the furious Lysiart. As Eglantine dies Euryanthe enters and rushes to Adolar. Lysiart is led off, and Adolar's sister finds peace at last because her ring was moistened by the tears of the innocent Euryanthe.