Idomeneo, King of Crete, after the siege of Troy, has wandered a long way from his home, where his son, Idamante, grown to man's estate during his absence, awaits him in filial love. Electra, daughter of Agamemnon, banished by the people of Argus on account of the matricide of Orestes, has taken refuge with Idamante, and becomes deeply enamoured of him. But Ilia, daughter of Priam, who, with other Trojan captives, has been sent to Crete by Idomeneo, has conceived a passion for Idamante, which he returns. At the opening of the opera we find Ilia struggling with her love for the enemy of her fatherland (aria, 2). Idamante approaches her joyfully. He has received tidings that his father's fleet is in sight, and has sent his old confidant, Arbace, to bring more exact intelligence. On this joyful day he gives freedom to all the Trojan captives, and declares his love for Ilia, which she, although reluctantly, rejects; whereupon he bewails himself in an aria (3). The captive Trojans are led in and loosed from their fetters, {IDOMENEO.} (128) giving occasion for a joyful chorus. Electra comes and expresses dissatisfaction at the liberation of so many enemies. Then follows Arbace with intelligence (which is mistaken) of the shipwreck of Idomeneo. Idamante departs overwhelmed with grief. Electra remains behind and gives vent to her jealousy and despair in a song (aria, 5). The scene changes to the sea-coast, and the fleet of Idomeneo is seen threatened by a storm, and driven on to the rocks, the mariners lamenting and beseeching aid. Neptune appears and commands the winds to depart. Idomeneo prays for his help, but the god casts threatening glances on him, and disappears. The sea being calmed, Idomeneo lands and declares that, during the storm, he has vowed to sacrifice to Neptune the first person who shall meet him on shore. He trembles at the rashness of his vow, and anxiously looks for the sacrifice he is to make (aria, 6). Idamante enters, having sought solitude as ease to his grief. He offers shelter to the stranger, whom he fails to recognise. In the course of conversation it transpires that he is mourning for his father Idomeneo. Whereupon Idomeneo makes himself known, but overcome by the horror of his situation, he departs, forbidding Idamante to follow him. The latter, ignorant of the cause, is inconsolable at his father's rejection of his proffered love and services (aria, 8). An intermezzo of suitable character follows the first act. The warriors of Idomeneo disembark to a march (9), are welcomed by their wives and children, and "express their joy in a grand figure-dance, ending with a chorus (10)."