The scene of action is alternately Memphis and Thebes and the story belongs to the period when the Pharaohs sat on the throne.
In the first act we see the King's palace at Memphis. Ramphis, the Highpriest of Pharaoh announces to the Egyptian General Radames, that the Ethiopians are in revolt and that the goddess Isis has decided who shall be leader of the army sent out against them. Radames secretly hopes to be the elected, in order to win the Ethiopian slave Aïda, whom he loves, not knowing that she is a King's daughter.
Enter Amneris, daughter of Pharaoh. She loves Radames without his knowledge and so does Aïda. Amneris, suspecting this, swears to avenge herself, should her suspicion prove correct.
The King's messenger announces, that Amonasro, the Ethiopian King (Aïda's father), is marching to the capital, and that Radames is chosen to conquer the foe. Radames goes to the temple [a] to invoke the benediction of the goddess and to receive the sacred arms.
In the second act Amneris, in order to test Aïda's feelings, tells her, that Radames fell in battle, and finds her doubts confirmed by Aïda's terror. Amneris openly threatens her rival, and both hasten to receive the soldiers, who return victorious. In Radames' suite walks King Amonasro, who has been taken prisoner, disguised as a simple officer. Aïda recognizes her father, and Amonasro telling his conqueror, that the Ethiopian King has fallen, implores his clemency. Radames, seeing Aïda in tears, adds his entreaties to those of the Ethiopian; and Pharaoh decides to set the prisoners free, with the exception of Aïda's father, who is to stay with his daughter. Pharaoh then gives Amneris to Radames as a recompense for his services.
In the third act Amonasro has discovered the mutual love of his daughter and Radames and resolves to make use of it. While Amneris prays in the temple that her bridegroom may give his whole heart to her, Amonasro bids his daughter discover the secret of the Egyptian warplans from her lover. Amonasro hides himself, and Aïda has an interview with Radames, in which he reveals all to her. She persuades him to fly with her, when Amonasro shows himself, telling him that he has heard all and confessing that he is the Ethiopian King. While they are speaking, Amneris overtakes and denounces them. Amonasro escapes with his daughter, Radames remains in the hand of Ramphis, the Highpriest.
In the fourth act Radames is visited in his cell by Amneris, who promises to save him from the awful death of being buried alive, if he renounces Aïda. But Radames refuses, though she tells him, that Aïda has fled into her country, her father being slain on their flight.
Amneris at length regrets her jealousy and repents, but too late! Nothing can save Radames, and she is obliged to see him led into his living tomb. Amneris curses the priests, who close the subterranean vaults with a rock. Radames, preparing himself for death, discovers Aïda by his side. She has found means to penetrate into his tomb, resolved to die with her lover.
While she sinks into his arms, Amneris prays outside for Radames' peace and eternal happiness.