Radames, a captain of the guard, is in love with Aida, a slave girl. Aida’s mistress, the King of Egypt’s daughter, Amneris, is also in love with the captain. Unknown to anyone in Egypt, Aida is an Ethiopian princess and the daughter of Egypt’s worst enemy, King Amonasro.
Ramfis, the High Priest, is on his way to inform the Egyptian King of the name of the general whom the goddess Isis has chosen to lead the Egyptians against the Ethiopians. Radames hopes to be chosen and, envisioning a glorious victory, expresses his affection for Aida. Amneris joins him. While she is questioning him, Aida enters. Noting that Radames is strongly affected by the appearance of Aida, Amneris suspects the reason for his embarrassment and is overcome with jealousy. Accompanied by his ministers, the King enters. A messenger is brought forward and confirms the Ethiopian invasion. Radames is announced as the chosen commander to lead the Egyptians against the enemy. Everyone pays homage to the young warrior and wishes for his victorious return. Aida, too, is caught up in the battle cry, and after the court leaves, berates herself for having betrayed her own people. Divided between loyalty to her father and country and her love for Radames, she asks the gods for strength. In the Temple, a solemn ceremony is held to prepare Radames for battle. He is presented with the sacred sword of Egypt.
The Egyptian troops led by Radames have won the war. Amneris, still tormented by doubt and jealousy, resolves to question Aida and confirm her suspicions. Amneris manages to shake Aida’s composure and tricks her into revealing her love for Radames. Amneris is furious and upset by the truth. The people celebrate the return of the victorious troops and their leader Radames, who asks that the Ethiopian prisoners be brought forth. Among them, Aida recognizes her father. Hiding his true identity, Amonasro pleads for the lives of his people. The Egyptian King accedes to Radames’s wish that the prisoners be set free. Ramfis, warning of the consequences, succeeds in having Aida and her father retained as hostages. In token of Egypt’s gratitude, the King awards Radames the hand of Amneris.
To prepare for her wedding to Radames, Amneris retires to the Temple of Isis to worship with Ramfis. Outside the Temple, Aida waits for Radames. Having given up on her own happiness, she recalls her childhood in the valleys of Ethiopia. Amonasro joins her and raises her hopes for a happy life at the side of her beloved. The Ethiopian captives who were freed have banded together and once again ready themselves to attack Egypt. Hoping to exploit Aida’s love for Radames, Amonasro proposes that she ask Radames the route the Egyptian armies will take. At first Aida refuses, but Amonasro soon crushes her resistance. Amonasro hides as Radames appears, still affirming his love for Aida and hoping another victory will allow him to win her once and for all. Aida does not share his enthusiasm and instead persuades him to flee the country with her. As they start to leave, Aida asks which route the Egyptian troops will take. As Radames answers her, Amonasro reveals himself and Radames realizes he has been tricked into giving up an important military secret. Realizing they have been overheard by Ramfis and Amneris as they leave the Temple, Amonasro and Aida flee and Radames surrenders to the High Priest, ready to accept the consequences of his betrayal. Amneris, torn between love and hatred for Radames, at last resolves to save him. She urges him to defend himself, but he refuses. The priests assemble and three times allow Radames a chance to present his defense. Three times he refuses and is sentenced to die. Amneris pleads with the priests to revoke the sentence.
In the darkness of a tomb, Radames is joined by Aida who had hidden there earlier. While the priests chant their hymns, the two lovers, at last united, spend their final moments in daydreams. Outside the tomb, Amneris asks forgiveness for her rancor and prays to Isis for redemption.