Ilia, daughter of the Trojan King Priam, reflects that her love for the Cretan prince Idamante, son of ldomeneo, outweighs her hatred for the conquerors of Troy, who hold her in bondage. Idamante, after declaring his affection for Ilia and his innocence, announces to the populace that in honor of his father’s return, the Trojan prisoners will be set free. There is general rejoicing.

Agamemnon’s daughter, the Greek princess Elettra, herself in love with Idamante, observes the growing affection between him and Ilia, swearing vengeance on her rival.

The king’s confidant, Arbace, brings the news that Idomeneo’s ship has been sunk near the coast; the king is believed dead.

On a deserted shore, ldomeneo regrets the terrible vow he has made to Neptune – in return for being spared, he must sacrifice the first living creature he encounters. Idamante comes upon the shipwrecked king, but recognition is slow after their long separation. When ldomeneo realizes the youth is his son, he rushes off, ordering Idamante never to seek him out. Idamante laments his father’s rejection, as citizens praise Neptune for the return of the king.

Idomeneo confides in Arbace, who suggests he send Idamante abroad. The king decides his son will escort Elettra home to Argos.

When Ilia confesses to ldomeneo that she has found a new father in him, he realizes his deliverance has cost Ilia’s happiness as well as his own. Though saved from the sea he still feels the rage of Neptune. Elettra, seeing in Idomeneo’s decision the prospect of requited love, expresses her hope.

As the ship to take Idamante and Elettra away stands ready in port, ldomeneo joins the pair in bidding farewell, but a sudden storm prevents the ship’s departure. Idomeneo confesses his guilt, without naming the required victim, and offers himself as the sacrifice.

Ilia longs for Idamante, who appears and tells her he will fight the monster sent by Neptune to ravage Crete. They reaffirm their love. Idomeneo and Elettra arrive. The king cannot bring himself to explain Neptune’s anger and again repudiates Idamante. Elettra’s jealousy is once more aroused. Idamante sadly takes his leave.

Arbace reports that the people, led by the High Priest, are demanding to see the king. Arbace pleads with the gods for pity. The monster has laid waste to the country: devastation and death reign on the island. The High Priest exhorts ldomeneo to reveal the identity of the victim Neptune demands. When the king names his son, the populace is horrified.

Idomeneo leads a prayer in the temple of Neptune. Preparations for the sacrifice are interrupted by rejoicing: Idamante has killed the monster. Now aware that ldomeneo has been shunning him out of love, Idamante begs his father to fulfill his vow. As Idomeneo prepares to strike, the oracular voice of Neptune resounds, commanding Idomeneo to renounce the throne in favor of Idamante, who will rule with Ilia as queen. Everyone expresses relief except Elettra, who invokes the furies and threatens suicide before collapsing. Peace is restored as ldomeneo presents Idamante to the people as their new king. Idamante’s alliance with Ilia and accession to the throne are met with rejoicing.