ACT I: In the antechamber of the royal palace at Memphis, Radames, a young captain of the guard, asks the high priest Ramfis if the goddess Isis has chosen a new commander to lead the Egyptian forces against the Ethiopians. Left alone, Radames hopes that he is the chosen one, imagining a glorious victory so he may free his beloved Aida, slave of Amneris, the King's daughter ("Celeste Aida"). Amneris, who loves Radames, interrupts his musing and questions him shrewdly; her suspicion that he loves Aida increases at the entrance of her slave girl (Trio: "Vieni, o diletta"). In the throne room, the King gives audience to a messenger from the front, who reports that the Ethiopian army, led by Amonasro, is marching on Thebes. The King announces the appointment of Radames as Egyptian commander and leads the assemblage in a battle hymn. As Radames is led off amid general rejoicing, Aida remains alone, appalled that she too has cheered him to victory ("Ritorna vincitor!"), for the Ethiopians are her people, and Amonasro, their king, is her father. Torn by conflicting loyalties, she begs the gods for pity.
In the temple of Ptha, a priestess is heard addressing the god as Radames is ceremonially clothed in the sacred armor and consecrated to the service of the god. All present invoke Ptha's guidance and protection for the army.
ACT II: Amneris is prepared by her attendants for the triumphal return of Radames, and is entertained by her slaves. At Aida's approach she dismisses her attendants and, hoping to confirm her suspicions that the slave loves Radames, tells her first that he has perished in battle, and then that he still lives (Duet: "Fu la sorte dell' armi"). At Aida's joyous outcry, Amneris threatens her and leaves for the festivities.
At a public square in Thebes a crowd welcomes the army, which passes in review before the King and Amneris (Chorus and triumphal march: "Gloria all' Egitto"). The spoils of war are carried past, heralding the arrival of Radames. Amneris presents him with the Shebbiu Collar, symbolizing his victory. Next, the Ethiopian captives are led in; among them Aida recognizes her father. Whispering to Aida not to disclose his rank, Amonasro pleads for the prisoners (Ensemble: "Ma tu, Re"). Though Ramfis and the priests demand the captives' death, Radames intercedes on their behalf; Amonasro, as their spokesman, is held hostage, but the others are set free. When the King gives Radames the hand of Amneris, Aida falls disconsolately into her father's arms.
ACT III: Ramfis welcomes Amneris to the temple of Isis on the eve of her wedding. While waiting for a meeting with her lover, Aida, overcome with nostalgia, mourns her lost homeland ("O patria mia"). She is jolted from her reverie by Amonasro, who demands that she learn Radames' plan for his new campaign against the Ethiopians (Duet: "Rivedrai le foreste imbalsamate"); she reluctantly agrees. Amonasro hides when Radames arrives and professes his love for Aida and his intention of asking for her as the prize for his victory against the Ethiopians who have once again attacked Egypt. She instead urges immediate flight and paints an evocative picture of their future happiness. (Duet: "Fuggiam gli ardori inospiti"). Winning Radames to her idea, she asks what route his armies will take so as to avoid them. No sooner does he reply than Amonasro emerges from hiding, reveals his identity, and urges Radames to flee with them. Amneris, overhearing, denounces Radames as a traitor. Amonasro attempts to kill her, but Radames blocks his path. Then as Aida and her father flee, he surrenders himself to the high priest.
ACT IV: In the forecourt of the hall of judgment, Amneris, bewailing the treason of Radames, determines to make a last effort to save him. When the guards lead him in, she offers to save his life if he will renounce Aida (Duet: "GiÓ i sacerdoti"); he refuses. Enraged, Amneris sends him to his doom but is immediately overcome with remorse. She listens in despair as his condemnation echoes from the hall of justice, and when the priests file out she curses them.
Radames, sealed in a vault beneath the temple, muses on Aida. He is startled by a noise: it is Aida herself, who has chosen to share his fate. Radames vainly tries to dislodge the stone that seals the tomb. Bidding farewell to earth, the lovers greet eternity (Duet: "O terra, addio"), while in the temple above, Amneris prays to Isis for peace.