Ramfis, the High Priest, and Radamès, a young captain of the Guard, are alone in a room of the palace. Radamès is hoping to be chosen commander of the Egyptian troops which must confront the invading Ethiopian hordes. Absorbed in his own dream of glory, Radamès longs for a great triumph he can offer his beloved Aida, an Ethiopian slavegirl in the service of Amneris, daughter of the Pharaoh. The priest must consult the goddess Isis to know the name of the one chosen to be the saviour of the fatherland. Amneris –also in love with Radamès– enters with Aida. Amneris is burning with jealousy as she knows Radamès loves another woman and houses the suspicion that the woman is Aida, her slave. The Pharaoh and priests arrive: Isis has determined that it is Radamès who will lead the Egyptian force against the invaders, led by their king Amonasro, father of Aida. On hearing of the appointment of Radamès, the slavegirl’s feelings are divided between love and patriotism. Inside the temple of Vulcan, we hear the distant chanting of the priestesses invoking the protection of the gods; blending in with them are the invocations of Ramfis and his priests. In the course of the ceremony that follows, Radamès is invested by the High Priest with the sacred arms and consecrated for victory, as all pray to the god to protect and assist him.
A group of slavegirls entertain Amneris as she prepares for the celebration of triumph, since news has been received of Radamès’s victorys. However, Amneris takes recourse to a scheme that will assure her of Aida’s feelings, giving the false news that the young commander has died in battle. The young slavegirl is incapable of hiding her distress, thus revealing that she truly loves Radamès. Amneris immediately admits she was lying and Aida cannot control her emotions, above all when Amneris confesses to her that she also loves Radamès. On the point of confessing her true origin, Aida manages to control herself and not mention her royal lineage, although she does admit that she lives only from and for the love of Radamès. Amneris threatens fierce revenge.
Triumphal scene at the gates of Thebes. The crowd breaks into joyous acclamations of victory. The parade is led by Radamès who, together with his troops, also leads their prisoners. Radamès receives the crown of victory from the hands of Amneris, and then begs clemency for the prisoners, among whom Amonasro is concealed. Seeing her father in chains, Aida cannot avoid revealing that she is his daughter. Ramfis advises the king to execute them but finally a compromise is reached that the prisoners will be freed on the condition that Amonasro remain as a hostage. The Pharaoh, as a prize for his victory, grants Radamès the hand of his daughter, Amneris.
Prior to the celebration of the wedding, Amneris and Ramfis go to pray at the temple of Isis. Aida appears in order to secretly meet with her beloved. However, Radamès’s interest toward his daughter has not escaped the attention of Amonasro who obtains from her a promise to find out from him the secret route that the Egyptian army will be taking. Radamès confesses that he will soon again be leading his troops and will request from the king as a prize for victory, freedom and the hand of Aida. She makes him see that neither the priests, nor Amneris, nor the Pharaoh will consent; only escape will put an end to their suffering. Radamès then reveals that the Nápata Pass will not be guarded and they can escape through it. Amonasro violently interrupts and reveals his true identity to Radamès, who is taken aback by the knowledge that he has unwillingly betrayed his country. They are surprised by Amneris and Ramfis as they leave the temple. Amonasro moves against Amneris, but Radamès intervenes, facilitating the escape of Aida and Amonasro, and then surrenders himself to the High Priest.
Amneris, still in love with Radamès, wants to save him, in spite of knowing he attempted to flee with Aida, and she tries to convince him to deny the charges against him, but to no avail. She then tells the general that Aida is alive and she will obtain a pardon for Radamès if he will renounce his love for the slavegirl. But he will not consent to it and prefers to atone for his own guilt. The young leader, judged by the court of priests, is found guilty and sentenced to the death reserved for traitors: he is to be buried alive. While Radamès waits his death, Aida appears unexpectedly. The Ethiopian has also had herself immured alive in order to end her days in the company of the man she loves.