In a hall in the royal palace at Memphis, the High Priest, Ramfis, tells Radames that the Ethiopians have invaded Egypt,and that the goddess Isis will soon declare who will lead the Egyptian army.Radames, hoping to be chosen, dreams of returning victorious to Memphis and to the arms of Aida, the captive Ethiopian slave whom he loves. Amneris enters and, seeing Radames' elation, suspects that his ardour is not just for military glory. She loves him herself, and her fears are reinforced when Aida enters and she detects the strong feelings between Aida and Radames.
The king arrives with Ramfis and the priests. A messenger tells of the devastation of the Egyptian countryside and of the threat to the capital, Thebes, from the Ethiopian armies under their leader Amonasro. No one knows of Aida's royal birth and her reaction to Amonasro's name goes unnoticed. The king proclaims that Isis has chosen Radames to lead the Egyptian army, and the assembled Egyptians sing a battle chorus with Amneris exhorting Radames to return victorious. Left alone, Aida repeats Amneris's words with bitter irony - she is torn between love for her father and country and her love for Radames.
The scene changes to the temple of Ptah where the high priestess, Ramfis and assembled priests and priestesses take part in a sacred ceremony to consecrate Radames' battle sword.
Radames has been victorious and Amneris and the ladies of the court sing in celebration. When Aida enters, Amneris decides to discover if her jealous suspicions are justified. She says that Radames has been killed in battle, and a devastated Aida is unable to hide her passion. Then Amneris scornfully reveals that he is in fact alive and victorious - and how dare a mere slave girl presumeto rival an Egyptian princess in love!
The king arrives in State and the court sings a chorus of praise and thanksgiving to Isis. Radames enters and on greeting him, the king offers to honour any request he makes. As the defeated Ethiopian prisoners arrive, Aida recognises Amonasro, her father amongst them. He tells her not to disclose his real identity and tells the Egyptians that the Ethiopian king is dead. He pleads for the lives of the prisoners and is supported by Radames and the Egyptian people. The priests and Amneris oppose the plea, but the king consents, retaining as hostages (at Ramfis's insistence) Aida and her father. As a reward for saving his country, the king offers Radames Amneris's hand in marriage - much to Amneris's delight and the distress of Aida and Radames.
Ramfis and Amneris approach the temple of Isis to pray for the goddess's blessing on her marriage to Radames, due to take place the next day. Aida arrives for a secret meeting with Radames and sings of her sadness at never seeing her homeland again. Amonasro appears and tells her that she could return home if she could only discover from Radames which route the attacking Egyptian army was planning to take. Initially she recoils at this idea, but confronted by her father's curses and contempt, she capitulates. Amonasro hides as Radames enters, and Aida persuades her lover to flee with her to Ethiopia, asking him as they part by which route they can avoid the Egyptian army. Radames discloses that the army will go through the Gorge of Napata, and on hearing this vital information, Amonasro steps from his hiding place and reveals himself as the Ethiopian king. Radames cannot believe that he has been led into betraying his country. Ramfis and Amneris, having witnessed the disclosures, emerge from the temple to arrest Radames. Amonasro attempts to kill Amneris, but Radames steps between them and tells Aida and her father to flee while he gives himself up to the guards.
Amneris is alone in a room close to the hall of justice where Radames' fate is being decided. She sends for Radames and offers to intercede on his behalf and save his life if he swears never to see Aida again. He refuses, and Amneris sends him to face his judgement, and in increasing distress she overhears his trial. He offers no answer to the charges of Ramfis and the priests, is condemned three times as a traitor, and is sentenced to be buried alive.In a crypt below the temple of Ptah, Radames is sealed in a living tomb. He hears a movement, and discovers that Aida has previously concealed herself in the crypt, choosing to die with him. As the lovers die in each other's arms, Amneris , in bitter isolation, prays for Radames eternal peace, whilst in the distance, the priests and priestesses chant in praise of the gods.