Thésée is a tragédie en musique, an early type of French opera, in a prologue and five acts with music by Jean-Baptiste Lully and a libretto by Philippe Quinault based on Ovid's Metamorphoses. It was first performed on 11 January 1675 by the Paris Opera for the royal court at the Château de Saint-Germain-en-Laye and was first performed in public in April at the Théâtre du Palais-Royal in Paris. The plot centres around a love triangle: Egée wants to marry his ward, princess Églé, while the sorceress Médée wishes to marry the young warrior Thésée, but Thésée and Églé love each other. Médée attempts to force the lovers to renounce each other: first by using her magic to bring Églé to a place of torment, then by convincing Egée to have Thésée killed as a potential threat to his reign. But before Thésée can drink the poison he has been given, Egée realises that Thésée is his lost son. He then gives Églé to Thésée. Médée takes vengeance by destroying the festive setting, but the goddessMinerve undoes this.
Princess Églé is in love with Thésée and prays for his safe return from battle against rebels who are threatening King Égée of Athens. Égée enters victorious. He tells Églé he is in love with her, despite being betrothed to the sorceress Médée. Égée says he now intends to marry Médée to his son, whom he has hidden away at Troezen and has not seen for years. The Athenians celebrate their victory with a sacrifice to the goddess Minerva.
Act 2
Médée is in love with Thésée. She agree to let Égée break off their engagement so he can pursue Églé. Égée is jealous of Thésée's popularity with the people of Athens, who want to make him the king's heir because of his bravery in battle. Médée offers to help Thésée, who reveals to her that he is in love with Églé, provoking the sorceress to jealousy.
Act 3
Médée threatens Églé that she will use her magic against her if the princess does not renounce her love for Thésée and marry the king instead. She conjures up a vision of a terrifying desert full of monsters and also menaces Églé with demons from hell.
Act 4
Médée orders Églé to marry Égée, otherwise she will put Thésée's life in danger. She conjures a vision of the sleeping Thésée in which she threatens to sacrifice him with a knife. Thésée wakes and is perplexed by Églé's sudden coldness towards him. She explains she is trying to save his life. Thésée reveals that he is Égée's son from Troezen. Médée appears to relent and bless the betrothal of Thésée and Églé.
Act 5
In reality, Médée is still tortured by jealousy. She persuades Égée to kill Thésée, warning him that if he makes Thésée his heir the king will wrong his missing son. Égée hands Thésée a poisoned chalice but he recognises Thésée's sword and realises the young man must be his son. Just in time, he prevents Thésée from drinking the poison. He agrees to let Thésée marry Églé. Thwarted, Médée escapes on a flying chariot pulled by dragons, burning down the palace. The Athenians pray to Minerva who raises a magnificent new palace and the opera ends with rejoicing.
