The Egyptian High Priest, Ramfis, tells the young officer, Radames, that the Ethiopians are again at war and a general has been chosen to command the forces against them. Left alone, Radames hopes he has been named general. He also hopes someday to marry the beautiful Ethiopian slave, Aida, and take her back to her own country.
The King’s daughter, Amneris, is also in love with Radames and, finding him alone, hints of her feelings. But when Aida joins them, Amneris quickly becomes jealous. Ramfis, the King and whole court enter. They hear an alarming report from a messenger: the Ethiopians are already invading, led by a fierce warrior named Amonasro. The King announces that Radames has been chosen to lead the Egyptians in battle, and calls on all Egyptians to defend the sacred Nile River. Amneris turns to Radames and instructs him to return victorious.
Aida remains alone. She repeats Amneris’ line ironically, and prays for the safety of her father, Amonasro. She then realizes that his victory would mean defeat for her beloved Radames, and gives a pitiful prayer for the gods to have mercy on her.
The priests are gathered for the ceremony of anointing Radames as General of the Egyptian armies. Priestesses sing a prayer and perform a ritual dance before the altar. Ramfis presents Radames with a sword and intones a solemn prayer for the protection of Egypt’s sacred soil. Radames joins in the prayer with all the other priests and the ceremony ends with an invocation to the Egyptian God, the Almighty Ptah.
Amneris’ female slaves tend to her toilette while singing the praises of Radames, who has led the Egyptian armies to victory. Aida enters. The Princess pretends sympathy for the girl because her people have been defeated, but her real purpose is to find out whether Aida is her rival for Radames’ love. She announces his death in battle, and Aida’s cry of anguish convinces the Princess it is as she has suspected. She accuses Aida, announcing that Radames is alive after all. Aida’s cry of “Thank God!” brings their rivalry out into the open. The slave begs for pity, but the Princess is passionately bitter. Suddenly, their exchange is interrupted by offstage trumpets and a chorus of triumph. Instructing Aida to follow her to the triumphal ceremonies, Amneris sweeps out, leaving Aida to repeat the pitiful prayers she had voiced earlier.
On a great avenue at the entrance to the city, crowds are gathered about the throne. The King mounts the throne, his daughter beside him. Finally the returning hero, Radames, is welcomed with the great Triumphal March. When the King offers him anything he wants as a reward, Radames’ first request is to have the captives brought forth. A miserable band of Ethiopians is brought in, led by their king, Amonasro. He manages to whisper to Aida not to betray his true identity. When he is asked to speak, he says that Amonasro has been killed; he is but a simple warrior. With great dignity he asks for mercy. The priests are against this, but Radames and the populace plead for the prisoners. A compromise is reached: all will be freed except Amonasro and Aida, who will be kept as hostages. Then the King announces that Radames, as his reward, shall marry the Princess. Amneris leads off Radames while Aida weeps.
It is the eve of Amneris’ wedding to Radames, and she must pray. Aida comes for a last rendezvous with her lover. She will bid him farewell and then drown herself in the Nile. She sings with longing of her native land. Before her lover arrives, Amonasro finds her and demands to know where to attack the Egyptian troops with his re-formed armies. Aida must get this information from her lover. She recoils in horror, but Amonasro is so eloquent in describing what defeat will mean for their people that finally she agrees. He hides as Radames nears. The lovers greet each other rapturously. Radames hopes the pending battle will delay his wedding to Amneris. Aida urges him to desert, but he refuses to turn traitor. She repels him, telling him to marry Amneris and forget her. Finally, he agrees to run away with her and tells her where the soldiers are stationed so they can be avoided. Amonasro, who has been waiting, rushes out, to the horror of Radames, and tries to drag off the young soldier with him. But suddenly Amneris and the Priest emerge from the temple. As Amonasro and Aida escape, the bitterly disillusioned Radames refuses to leave with them and, with a dramatic gesture, surrenders himself to the Priest.
Radames is to be tried for treason. Amneris waits nearby and demands that he be brought forth. When he comes, she pleads to be allowed to save him; all she wants is for him to give up Aida and marry her. Even when he learns that Aida has escaped and Amonasro has been killed, Radames turns down her offer, realizing he cannot live without Aida. He is led off by his guards. As the priests, solemnly chanting, file past to go to the hall where the trial will take place, Amneris reviles him and herself. Her jealousy, she says, will bring death to her beloved. The trial begins; Radames is charged with deserting camp before battle, betraying his country, his King and his honor. Though called on to do so, he makes no defense. His sentence is then pronounced: he is to be buried alive beneath the altar of the god he has failed to honor. When the priests have filed back from the hall of justice, Amneris curses them in her anguish. They remain unmoved, and she works herself into a wild fury of frustration as Radames is led into his tomb and it is sealed shut.
As Radames awaits death, he utters a soft wish that Aida may be happy and never hear of his fate. A moment later a figure approaches; it is Aida. Hearing of Radames’ sentence, she has managed to get into the tomb and has been waiting to die with him. With a cry of anguish he tries to lift the heavy stone, for he cannot bear the thought of her death, but death is already coming over her. Together they sing a last farewell to earth. Amneris kneels above the tomb and prays for peace as the curtain falls.