Act 1: The story centers on King Idomeneo of Crete, and takes place just after the Trojan War. Idomeneo is returning home after 10 years of battle. Allied with the Greeks, Idomeneo and his army have finally defeated the Trojans. As the action beings, Idomeneo is still at sea, but some of his forces have reached Crete and they've brought along a captured Trojan princess named Ilia.
Ilia is alone in Idomeneo's palace. She's grieving over her father and brother, who were killed during the war. She still hates Idomeneo, but she has fallen in love with the king's son, Idamante.
Idamante loves her as well, but while he admits his feelings, Ilia cannot — it was Idamante's army that killed her family. Trying to win her over, Idamante announces the release of all Trojan prisoners, and people begin to celebrate. An exception is the high-strung Greek princess Elektra, who is also in love with Idamante. She's jealous of Ilia, and objects to Idamante's generosity toward their Trojan enemies.
Idamante dismisses Elektra's complaints, but the celebration is interrupted again by Idomeneo's aide Arbace. He announces that the King's ship has been lost in a storm while approaching the harbor.
The scene changes to the seashore, where a terrified crowd prays for safety. When the storm subsides, the crowd scatters, and we finally meet the title character, King Idomeneo, stumbling out of the water after barely surviving the shipwreck. It seems that just as he was going down for the last time, the sea god Neptune offered to save him — on one condition. In return for his life, Idomeneo has agreed to sacrifice the first person he meets after reaching the safety of land.
In the sort of convenient coincidence that is typical of opera seria, that person is his own son, Idamante. At first the two don't recognize each other. When Idomeneo finally realizes whom he has promised to kill, he runs off in a panic, leaving Idamante thoroughly confused.
In the Salzburg production, the first act is followed by an Intermezzo that is often omitted from modern performances. There's an orchestral march, then the citizens sing an enthusiastic but ironic chorus. They sing in praise of Neptune — the god whose deadly wrath is about to threaten their kingdom.
Act 2:
Idomeneo is seeking counsel from his friend Arbace, who thinks Idomeneo can save Idamante's life by sending him into exile. If he can't be found, how can he be killed? They decide to send Idamante to Greece, along with Elektra.
Idomeneo then meets with Ilia. She has fallen completely in love with Idamante, and hopes that Idomeneo's miraculous return means that they can all mend their differences. This leaves Idomeneo more confused than ever — he has sworn to kill his son, and now this enemy princess is acting like they're all one big happy family. His turmoil leads to the brilliant aria Fuor del mar, in which Idomeneo says he'd have been better off if Neptune had let him drown! Still, he's going through with his plan to send Idamante into exile.
That's just fine with Elektra, who is more than happy to go off with Idamante and leave her rival Ilia behind. But Neptune catches on to the scheme and won't allow it. As the ship bearing Idamante prepares to leave, Neptune whips up another storm, and throws in an enormous, bloodthirsty sea serpent for good measure.
With Idamante's ship stuck in port, and his people terrified, Idomeneo finally admits to his subjects that the whole mess is his fault. But he doesn't tell them exactly what he's promised to the angry sea god. With Idomeneo in a quandary, his people run for cover.
Act 3:
The sea monster Neptune conjured has come ashore, and is laying waste to property and people. Alone, Ilia sings of her love for Idamante. He comes to her, but tells her he's leaving to do battle with the monster.
Before he goes, Idomeneo and Elektra arrive. When Idamante asks his father why he is to be sent into exile, Idomeneo gives him a disingenuous reply. The four characters then join in a quartet that may be the opera's finest number — four people expressing wildly contrasting emotions, through intertwining lines that create a seemingly impossible harmony.
In the next scene, Idomeneo's confidante Arbace appears. He reports that all hell is breaking loose in the countryside. To avoid disaster, Idomeneo must fulfill his promise to Neptune and sacrifice his son. Idomeneo gives in, telling the citizens what his deal with Neptune really is. They're horrified, but say a deal's a deal — nobody wants to be eaten by that monster! The ritual of sacrifice is prepared.
Just then, Idamante rushes in to say that he has killed the monster. No matter, he's told — Neptune is still demanding his life. For the first time, Idamante fully realizes what's been going on. Both he and Ilia bravely offer to sacrifice themselves. With so much selflessness on display, Idomeneo and the High Priest wonder whether they can somehow reach an agreement with the sea god.
As they're mulling things over, Neptune himself weighs in. He agrees to release Idomeneo from his promise, on one condition: Idomeneo must abdicate, leaving the rule of Crete to Idamante and Ilia. Everyone is thrilled and gives thanks to Neptune — everyone but Elektra, who delivers her furious aria, D'Oreste, d'Aiace. All she wants is to have Idamante for herself — and here she is dealing with a foolish King, a rival princess, an angry god and even a sea serpent! At the end of her aria, Mozart's stage direction reads, "parte infuriata," so Elektra leaves in a huff.
Free to continue their celebration, the people of Crete call on the god of love to bless Idamante and Ilia, and bring them all peace, as the opera ends.
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