Scene 1: The hall of the king's palace at Memphis
The high priest Ramfis tells Radames that the Ethiopians have invaded Egypt. He is on his way to tell the king the name of the general who has been named by the goddess Isis as leader of the defending army. Radames hopes that he has been chosen and will be able to lay the spoils of victory at the feet of his beloved Aida.
Amneris, daughter of the king, is in love with Radames, and tries to sound his feelings about her. He is evasive, and her suspicions are confirmed when she sees his reaction to the presence of her Ethiopian slave Aida.
In the presence of the king and people a messenger brings news of the Ethiopian invasion, advancing on Thebes under their king Amonasro. Announcing that Radames has been chosen by the goddess, the king instructs him to go to the temple of Vulcan to receive a consecrated sword. Amneris presents him with a banner and the people, led by the king, proclaim their defiance of the invader and invoke victory on the head of Radames. Aida involuntarily echoes the cry, but when she is alone, she is torn between her love for Radames and her love for her country.
Scene 2: The temple of Vulcan at Memphis
Priests and priestesses invoke the god Phtah (Vulcan). Radames prays the god to protect the sacred soil of Egypt and receives the sword.
Scene 1: The apartments of Amneris in the palace at Thebes
As she is adorned by her slaves for the celebration of an Egyptian victory, Amneris hopes that her love for Radames will be returned.
Pretending sympathy with Aida for the defeat of her people, she tricks her into revealing her love for Radames and declares herself her rival. Aida restrains herself, on the brink of revealing her equality of rank, and begs for mercy, while Amneris exults that Aida will have to grovel at her feet at the triumphal reception.
Scene 2: An entrance to the city of Thebes
Radames is borne in triumph through the gate. The king offers any reward he chooses and he asks for the prisoners to be brought forward. Aida recognises her father, who warns her not to betray his identity and then tells the king of Egypt that the Ethiopian king was killed in the battle.
As his reward Radames asks for the life and freedom of the prisoners, but Ramfis warns the king that they are powerful enemies. The king grants the request, but accepts the advice of Ramfis that Aida and her father should be kept as hostages. He awards Radames the hand of Amneris, saying that he will one day rule Egypt with her. Amneris is triumphant, Radames thunderstruck and Aida downcast, while her father tries to encourage her with promises of vengeance.
The banks of the Nile near the temple of Isis
Accompanied by Ramfis, Amneris has come to the temple to pray for a blessing on her marriage to Radames.
Aida comes to bid farewell to Radames. She thinks sadly of the homeland she will never see again. Amonasro has been waiting for her and tries to persuade her to find out from Radames the route the army will be taking on a punitive expedition against the Ethiopians. When she refuses, he overwhelms her with a vision of her family and country laid waste and cursing her, adding the threat of his paternal curse - and she consents. He hides as Radames arrives.
Aida greets the passionate Radames coldly. She cannot believe, she says, that he will be able to extricate himself from the marriage to Amneris. If he really loves her, the only way out is for them to flee to Ethiopia. He is reluctant, but eventually yields, and she asks him which will be the safest way to travel, so as to avoid the Egyptian army. He replies that the army will go by the pass of Napata, and Amonasro emerges from the shadows, promising that his army will be there.
Radames realises that he has unwittingly betrayed his country, and refuses to be comforted by Aida and Amonasro or to flee with them. Amneris and Radames emerge from the temple, accusing him of treachery and Radames, foiling the attempt of Amonasro to kill Amneris, surrenders to Ramfis as Aida and Amonasro escape.
Scene 1: A hall in the palace
The remorseful Amneris hopes to save Radames, but he refuses her offer of help with its condition that he forget Aida, accusing her of being responsible for Aida's death. She answers that athough Amonasro was killed in the battle, Aida's fate is unknown. Radames spurns Amneris' love and declares himself ready to die.
Amneris listens to the trial as Radames remains silent in the face of the accusations of Ramfis and is condemned to be buried alive. She tries in vain to plead with the priests for his life and curses them when they remain obdurate.
Scene 2: The interior of the temple of Vulcan with a subterranean hall beneath it
The entombed Radames regrets only that he does not know Aida's fate. But she has learnt of his sentence and crept into the tomb to die with him. As they bid farewell to life, Amneris, in the temple above, prays for Radames.