The goddess Isis has chosen a general to lead the Egyptians against the Ethiopians. Ramfis, the High Priest, is on his way to inform the Egyptian King of the goddess’s choice. Radames hopes to be chosen and, envisioning a glorious victory, expresses his secret affection for Aida, a captured Ethiopian woman. Princess Amneris, who loves Radames, notices that he is strongly affected by Aida. Amneris is overcome with jealousy. Accompanied by his ministers, the King enters. A messenger brings news of the Ethiopian invasion. Radames is announced as the chosen commander to lead the Egyptians against the enemy. The crowd expresses their hopes for his victorious return. Aida, too, is caught up in the battle cry, and after the court leaves, berates herself for having called for the defeat of 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 trick Aida into revealing her love for Radames; furious, she tells Aida to learn her place and give up all hope of winning Radames away from her. The people celebrate the return of the victorious army. Radames asks that the Ethiopian prisoners be brought forth; among them, Aida recognizes her father. Hiding his true identity, King Amonasro of Ethiopia 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 demands she find out from Radames the name of the route the Egyptian armies will take. Amonasro conceals himself nearby, where he can overhear the plan. Radames affirms his love for Aida and hopes 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. Radames answers her, whereupon Amonasro reveals himself and Radames realizes he has blurted out an important military secret. Realizing they have been overheard by Ramfis and Amneris as they leave the Temple, Amonasro and Aida flee. Radames surrenders to the High Priest, ready to accept the consequences of his betrayal.
Torn between love and hatred for Radames, Amneris 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; but three times he is silent. They sentence him to death. Amneris pleads with the priests to revoke the sentence, and when they ignore her, she curses them.
Buried alive in a tomb, Radames finds Aida, who had hidden there earlier. While the priests chant their hymns, the two lovers die, united at last. Above their tomb, Amneris prays to Isis for peace.