In the royal palace at Memphis the high priest Ramfis tells the young warrior Radames that Egypt is threatened once again by the barbaric Ethiopians and that the goddess Isis has named the Egyptian supreme commander. Left alone, Radames wishes he would be the one chosen, and he muses on his love for the slave girl Aida. Princess Amneris desires Radames herself and suspects she has a rival. When Aida enters, the girl's anxiety betrays to the observant Amneris who this rival is. The King arrives with his retinue. A messenger tells about border attacks by marauding Ethiopians led by their king Amonasro. The King reveals that Radames is to be supreme commander of the Egyptian forces; he is to go to the Temple of Ptah for consecration. Aida is horrified by having wished her lover victory over her own father and people. Torn by conflicting loyalties, she prays to the gods for mercy. Ramfis entrusts the sacred sword to Radames, and all pray to Ptah to bless his mission.
Slave girls are grooming Amneris for the celebrations in honour of Radames' victory. Languishing with love, she dreams of his return. With feigned sympathy for Aida, she tricks the slave girl into revealing her love for Radames by pretending he has fallen in battle. Then she admits her lie, and threatens Aida as a rival.
The Egyptian army parades by the gates of Thebes. The King says Amneris will crown Radames victorious. When offered whatever reward he wants, Radames asks the prisoners to be brought in. Aida recognizes her father among them; he warns her not to betray that he is the Ethiopian king. Ramfis and the priests insist that the dangerous Ethiopians are stamped out for all, but Radames asks mercy for them. Ramfis requests that Aida and her father, at least, be kept as hostages. Accepting this compromise, the King announces the marriage of Radames and Amneris.
Amneris comes by boat to the Temple of Isis with Ramfis, who tells her to pray for the goddess' favour toward her marriage. Then Aida appears, waiting to meet Radames at the banks of the Nile. She thinks lovingly of her homeland. Amonasro springs out of the shadow, telling her she can defeat her rival and be free to see her home again if she asks Radames what road the Egyptian troops plan to take in their next campaign. Horrified at first at the idea of such treachery, she finally agrees when Amonasro depicts her the inevitable destruction of her country. Amonasro hides as Radames appears. Aida suggests they flee together to her native land. When he eventually agrees to her plan she asks what road will be the safest to avoid the troops. He replies the troops will pass through the gorge of Napata, whereupon Amonasro jumps forth triumphantly. Radames recognizes what he has done. Looming out of the temple with Amneris, Ramfis orders Radames arrested. Amonasro escapes with Aida.
Radames is led into a hall in the palace to face judgement in a nearby chamber. Amneris urges him to defend himself, saying she will use her influence to save him if he will marry her. He refuses saying that life has no meaning for him without Aida. The priests within read the charges against Radames, but he says nothing to defend himself. They fend him guilty of treason and condemn him to be entombed alive "below the altar of the outraged god". The priests in the Temple of Ptah seal the heavy stone over the tomb in which Radames has been placed. Thinking of Aida, he discerns her beside him in the darkness: she has chosen to share his fate. They bid farewell to earth as Amneris, kneeling above the tomb, prays for peace.