Act One
In the royal palace at Memphis, the high priest, Ramfis, informs Radames, a young captain of the guard, that Ethiopia is posing a threat to the Nile Valley. Radames hopes that the goddess Isis will choose him to be the leader of the Egyptian army, believing that a victory over the Ethiopians would enable him to free his secret love, Aida, the Ethiopian slave of the king's daughter, Amneris. Amneris, who also loves Radames, enters and begins to question Radames, suspecting that he is in love with Aïda. A messenger arrives to announce that Amonasro is leading the Ethiopian army to march on Thebes. Radames is appointed leader of the Egyptian army, and he leads his men in a battle hymn ("Su! del Nilo!"). Amneris is echoed by her people as she cries, "Return Victorious!" Aïda is stunned to hear herself repeat these words, and is left to contemplate her conflict of loyalties. Although her captors do not know it, she is the Princess of Ethiopia and her lover will soon be engaged in battle with her father.

Act Two
Radames is the victor in the battle. While awaiting his return, Amneris is groomed and entertained by her slaves. In an attempt to determine if Aida does indeed love Radames, Amneris tells her slave that he was slain in battle ("Fu la sorte degli armi"). Aïda's sorrowful response reveals her secret love. Amneris then discloses that Radames is really alive, but subsequently threatens Aïda, She is to keep her place as a slave and ignore her feelings for Radames. Aïda nearly confesses her royal identity, but instead, pleads for mercy. As Radames returns, a parade and dances celebrate the success of Egypt ("Gloria all' Egitto, ad Iside"). Radames is crowned victor by Amneris and his captives are lead in; these include Aïda's father, Amonasro, disguised as an officer. He warns Aida not to give away his royal identity, and proceeds to plead for mercy for the lives of his fellow people ("Ma tu, Re, tu signore possente"). Ramfis and the priests suggest that the captives be killed, but Radames asks for the captives' freedom to be his reward. Ramfis suggests that all but Aida's father be released. The King agrees and gives Radames the hand of Amneris as his reward for victory.

Act Three
Ramfis leads Amneris to a temple of Isis on the bank of the Nile to receive a blessing on the eve of her wedding. Her face veiled, Aïda enters to wait in secrecy for Radames. She is immersed in nostalgic thoughts as she longs for her conquered homeland ("O patria mia"). Her thoughts are interrupted by the appearance of her father, who has learned of her love for Radames. He encourages her to betray Radames by tricking him into revealing the Egyptian army’s invasion plan. She attempts to decline, but finally agrees as he scolds her loyalties and reminds her of what the Egyptians have done to her beloved homeland. Unaware of Amonasro's presence, Radames appears to Aïda, declaring that he will marry her after his next victory. She instead insists that they run away together to Ethiopia ("Fuggiam gli ardori inospiti"), and asks which route they will take. Upon hearing the Egyptian plan, Amonasro shows himself and declares that he is actually the King of Ethiopia. Radames is horrified by his unwilling act of treason ("Io son disonorato!"), while Aïda and Amonasro try to convince him that it was an act of fate. Amneris discovers them, declaring that Radames must be a traitor. Amonasro lunges at Amneris with a drawn dagger, but he is stopped by Radames. Radames unexpectedly urges Amonasro and Aida to escape. He then places himself under arrest ("Sacerdote, io resto a te").

Act Four
Radames is sent to the Temple of Judgement where Amneris offers him a chance to save himself. She will plead for him if he will forsake Aïda ("Già i sacerdoti adunansi"), yet he refuses, preferring death. Amneris listens as the priests ask him three times to defend himself. Radames refuses to answer and is condemned to death. Amneris, appalled at the consequences of her jealousy, declares that their need for revenge will kill an innocent man. Buried alive in a tomb beneath the temple, Radames thinks only of Aïda. Suddenly, she appears in the presumably sealed temple basement, having slipped in earlier to share her lover’s fate. Radames fails in a final, desperate attempt to remove the stone holding them in the chamber. Resigned to their fate, the lovers bid farewell to the earth ("O terra addio"). Radames faces death with Aïda. Appearing above the vault, Amneris mourns and prays.