In a hall of the King’s palace, the high priest Ramfis and the warrior Radames discuss Ethiopia’s imminent invasion of Egypt. Ramfis leaves to deliver to the King the name of the general whom the goddess Isis has chosen to lead the Egyptians. Alone, Radames dreams of being selected and returning to Egypt as a glorious conqueror, dedicating his victory to the woman he loves, the Ethiopian slave, Aida. Amneris, the King of Egypt’s daughter, lusts after Radames. She encounters him and asks why he appears so transported. He skirts her probing questions, but when Aida enters, his reaction triggers Amneris’ suspicions.
The King arrives and listens to a messenger explain that Ethiopian troops led by their leader Amonasro, have attacked Egypt. The King announces that Isis has chosen Radames to lead their army. Radames leaves to go fight the Ethiopians. Aida, although now a slave, is in reality a Princess of Ethiopia and the daughter of Amonasro, but she joins in wishing Radames victory. She realizes what she has said and is horrified, torn by her love for him and for her country. She prays for mercy.
Under Radames’ leadership, the Egyptian troops have been victorious. Amneris prepares for the Egyptian army’s triumphant return. Amneris tricks Aida by telling her that Radames was killed in battle. Aida’s anguished reaction fuels Amneris suspicion. When Amneris then admits that Radames is really alive, Aida cannot conceal her joy. Amneris declares herself a rival, prompting Aida to ask for pity. Amneris vows revenge and ill-fortune on Aida.
In a triumphal parade, Radames and his soldiers march into the city. When the Ethiopian captives are brought before the King, Aida is shocked to recognize her father in the throng. The King presses the man for his identity. He admits to being Aida’s father, but says that King Amonasro perished in battle. He begs for clemency for the Ethiopians. Impressed by Amonasro’s eloquence, Radames asks as his reward that the priests’ death sentence on the prisoners be overruled and that they be freed. The King spares the prisoners’ lives, but keeps Amonasro as a hostage. He publicly awards Radames his daughter’s hand in marriage. Aida is crushed and Radames horrified, and Amneris believes that she has defeated her rival.
On the banks of the moonlit Nile, Amneris and Ramfis come to worship in the temple of Isis, on the eve of her wedding. Aida has arranged to meet Radames near the same temple. Aida dwells on the conflict between her country and her love.
Amonasro follows Aida and asks her to trick her lover for the sake of the Ethiopians. He tells her that the Ethiopian armies have regrouped and wait to attack again; they need only to know where the Egyptian army will move. Aida is appalled, but Amonasro squashes her resistance by taunting her with visions of her dead mother and the destruction of Ethiopia. When he hears Radames approach, Amonasro conceals himself.
Aida entreats Radames to flee with her back to Ethiopia. .Hesitantly, he agrees. As they begin their departure, Aida asks him about the Egyptian troop movements, ostensibly so that they can evade them. Radames reveals the information. Amonasro emerges from his hiding place and reveals himself as the King of Ethiopia, promising Radames the hand of Aida for his help. Amneris hears this, and comes from the temple calling Radames a traitor. Amonasro attempts to kill Amneris, but is stopped by Radames. Radames helps Amonasro and Aida flee, but remains behind and surrenders his sword to the High Priest.
Wild with love for Radames and anger at his actions, the princess summons Radames. If he will swear to give up Aida, she tells him, she will intercede with her father and save his life. He refuses and is taken to the temple for his trial. In the hall of justice, Radames is changed with treason. Three times he is asked to defend himself, and three times he refuses to speak on his own behalf. The priests sentence him to be entombed alive. Hearing the verdict, Amneris is enraged at the hypocrisy of the priests, and curses them.
The stone slab under the altar of the Temple of Thtah is hammered closed over Radames. To his amazement, he discovers Aida. Knowing his fate, she has chosen to join him, finally, in death. Sounds of the priests’ ritual dances drift through the walls as the two lovers vow their devotion to one another. They bid farewell as they wait for death, together at last. Outside, a grieving Amneris prays to the Goddess Isis to grant Radames eternal rest.