A herald proclaims to the crowd of despairing citizens of Thessaly that King Admète is close to death. Queen Alceste voices her own sorrow, then leads the people to the temple to offer a sacrifice to Apollo and to plead for the king’s life. In the temple, before a statue of Apollo, the High Priest prays for the king. At the prayer’s climax, the High Priest announces that Apollo is present. His oracle speaks through the statue, declaring that the king will die that same day if no one is willing to die in his place. The people flee, leaving Alceste alone. She resolves to save her husband. The High Priest accepts her vow and tells her that before night falls, her husband will recover and she will die.

The citizens celebrate the recovery of their king. When Admète asks how his health has been restored, he is informed of the oracle’s decree and of the anonymous sacrifice. Admète is shocked, and rejects the sacrifice, declaring that he would rather give his own life. Alceste enters, and while she struggles to hide her sorrow, Admece questions her incessantly about the unknown victim. The truth is unveiled and Admète refuses to accept life on such terms, threatening to annul her vow by killing himself. When Admète leaves, Alceste reaffirms her decision to die in his place.

Évandre, a leading citizen of Thessaly, leads the people in mourning. The mythical strongman Hercule appears, having paused during the course of his celebrated labors. Évandre explains that Alceste has already made her way to the altar of death, followed by Admète. Hercule declares his intent to rescue her. Admète joins Alceste at the entrance to the underworld. Alceste urges him to live for the sake of his children and his subjects, but he rejects her arguments. Each competes in offering to die to save the other. An underworld god offers Alceste one last chance to rescind her offer. She refuses, and is at the point of dying when Hercule enters. He drives back the spirits, and restores Alceste to her husband. The god Apollo appears. He rewards Hercule with the promise of immortality and commands Admète and Alceste to live together as a model of marital devotion.