The high priest Ramfis informs Radamès, the captain of the guards, that the Ethiopians have rebelled and are threatening the Nile valley. The goddess Isis has just told him the name of the man who is to command the Egyptian army, and the high priest goes off to inform the King. Left alone, Radamès cherishes the secret hope that he himself has been selected and revels in the thought of his glory and above all of Aida, the Ethiopian slave whom he passionately loves. The expression on his face does not escape the notice of Amneris, the King’s daughter, who secretly loves him and fears a rival. When Aida enters, Radamès’s agitation and that of the slave herself heighten Amneris’s suspicions. Preceded by guards, ministers and priests, the King enters. A Messenger tells of the invasion by the Ethiopians who, led by their king Amonasro, are about to attack Thebes. The King informs the assembled company, who invoke war, that Isis has chosen Radamès to command the army. Radamès exults. Amneris hands him a flag and everyone exhorts him to return as the winner. Aida alone is desperate. For she cannot hope to see her people defeated and Amonasro, her father, dragged in chains, or to lose her beloved Radamès.
In the interior of the temple of Vulcan at Memphis, Ramfis and the priests stand beneath the altar. From within is heard the song of the priestesses invoking the god Fthà. Radamès is ushered into the temple. While the priestesses dance, a veil is spread over the army chief’s head and he is invested with the sacred arms. Ramfis entrusts to him the fate of Egypt.
While Moorish slaves dance, Amneris has herself attired by her slave girls, in preparation for the triumphal festivities in honour of the conqueror Radamès. Aida enters, looking shaken. Amneris is suspicious and pretends to share her sorrow over the defeat of the Ethiopian people. And when she gives her the false news that Radamès has been slain in battle, Aida’s desperation confirms that the slave is her rival in love. The two women confront each other. In a fury, Amneris threatens Aida and forces her to attend her approaching triumph.
At an entrance to the city of Thebes, the King arrives with his retinue, and sits on the throne. Next to him are the princess Amneris with her slaves and Aida. The people sing the praises of Egypt, Isis and the King. With a fanfare, the victorious army files past the King. At the end of this triumphal procession comes Radamès. Amneris places a laurel crown on his head and the King invites him to ask for whatever he wishes. Aida catches sight of her father Amonasro, dressed as an ordinary officer, among the Ethiopian prisoners. She embraces him but, in order not to betray him, does not reveal his royal identity. Amonasro presents himself to the King and appeals for clemency to his conquered people. The Egyptians commiserate with the prisoners, but Ramfis and the priests advise the King to show no mercy. At this point Radamès intervenes to spare the lives and freedom of the defeated Ethiopians. The King consents and proclaims – on the advice of Ramfis – that Aida and her father alone shall be held, in token of peace. He then grants the hand of Amneris to Radamès, who shall one day rule over Egypt. Amneris’s exultance is echoed by Aida’s despair.
From the temple of Isis are heard songs in honour of the goddess. And off a boat step Amneris, Ramfis, veiled women, and guards. It is the eve of the marriage, and Amneris goes to the temple to pray. Aida enters, covered by a veil. She awaits Radamès, who has arranged a tryst with her there. In the meantime the girl muses on her frustrated dream of love and the beauties of her homeland never to be seen again. Her father Amonasro appears. Having noticed her feelings for Radamès, he puts before her the prospect of a return home and of happiness in love. But first the Egyptians must be defeated. He therefore asks her to obtain by fraud from Radamès the secret route his army will be taking. Aida is reluctant, but eventually yields to Amonasro’s insistence when he calls her a slave to the Pharaohs and threatens to repudiate her. Amonasro now hides. Radamès enters, and confides to Aida that he is hoping for another victory, which will enable him to obtain permission from the King to marry her as a reward. Aida proposes instead that he flee with her to Ethiopia, where they can live happily. Radamès hesitates, but she persuades him and they prepare to elope. Aida asks Radamès how they can escape the army, and he replies that until the next day the gorge of Nàpata will be safe. When Amonasro hears this name, he emerges and reveals that he is the king of the Ethiopians. Radamès is petrified and realises he has betrayed his country. When Amneris comes out of the temple and accuses Radamès of betrayal, Amonasro leaps to kill her. But Radamès steps between, and manages to prevent him striking her. He lets Aida and her father escape and gives himself up to Ramfis.
In a hall in the King’s palace, Amneris is desperate. Radamès is about to be tried for treason. She still loves him and is determined to do her utmost to save him. She has him brought into her presence and begs him to clear himself of the charges against him. Radamès refuses. He declares that his honor is untarnished and that he is resolved to die, having lost Aida. Amneris reveals that the girl is alive and free, and promises to have his life spared if he will renounce her. However, Radamès reiterates his decision. Amneris sees the priests go down to the dungeon to pronounce their sentence. Accused of treason, desertion and abuse of the King’s trust, Radamès refuses to defend himself and is condemned to be buried alive. In despair, Amneris curses the priests.
Two priests seal the dungeon in which Radamès is entombed. In the darkness he hears a cry and glimpses a figure moving towards him. It is Aida, who had crept in earlier, to die in the arms of her beloved. As the angel of death approaches, they bid farewell to earthly life and prepare for heaven. In the temple Amneris, in mourning, prays to Isis for Radamès’s soul.