The Royal Palace at Memphis; Egypt and Ethiopia are at war. Radames and Ramfis enter. They discuss the rumour that the Ethiopians are planning a new attack and Ramfis discloses that the priesthood has consulted the goddess Isis as to who should lead Egypt’s forces. He looks meaningfully at Radames, but names no-one. Alone, Radames muses on forthcoming glory and the chance that it may enable him to marry Aida, a beautiful Ethiopian slave girl. Amneris enters and asks about his evident happiness. She hints at a loved one in Memphis and he looks away. She is consumed by jealousy because she loves Radames herself. Aida enters and Amneris feigns kindness to her, while noticing that Aida and Radames cannot look at one another.
The Pharaoh enters with the court. A messenger brings news of the Ethiopians’ coming attack, led by King Amonasro who, unknown to everyone, is Aida’s father. There is a call for war. Radames is named general and given the standard by Amneris. The battle hymn concludes and the court departs. Aida is left alone in confusion, unable to pray for Radames’ victory, yet, in her love for him, wishing his safe return.
At the Temple of Vulcan a ritual is in progress. Radames is led in by Ramfis and consecrated with his weapons at the high altar. Both invoke the blessing of the god Ptah for the campaign.
In her apartments, Amneris awaiting the return of Radames, the man she loves and who has been victorious, and is entertained by dancers. Aida comes in with Amneris’ crown. Feigning kindness once again, Amneris hints that she knows Aida is in love with an Egyptian. She then casually mentions that Radames has been killed and Aida’s outburst reveals to her the truth. Admitting that she lied, Amneris cruelly threatens Aida who – after a moment’s defiance – vainly begs for forgiveness. Outside, the sound of the returning warriors is heard.
In Thebes, the Pharaoh, Amneris, priests and courtiers await the arrival of Radames and his victorious army. At the end of a long procession Radames enters and is crowned victor by Amneris while the Pharaoh proclaims him the country’s saviour. Asked to name a favour, Radames requests the Ethiopian slaves be brought in. Aida recognises her father, but Amonasro whispers to her not to betray his true identity. To the Pharaoh he describes how he and his comrades fought valiantly but how their King was killed in battle. He pleads for mercy and Radames asks that the captured Ethiopians be set free. Ramfis opposes him, but eventually consents so long as the “spokesman” is kept under custody. The Pharaoh agrees, and then promises Radames the hand of his daughter in marriage. Radames will reign after him. Only Radames and Aida privately voice sorrow at the royal decree.
At night on the banks of the Nile the sounds of chanting can be heard coming from the Temple of Isis. Amneris arrives with Ramfis to spend time before her wedding in prayer and they go into the Temple. Aida enters to meet Radames in secret. As she cannot be his, she thinks of drowning herself in the Nile. But Amonasro appears having eluded his guard, and tells Aida that she must help her country in a new uprising. She must find out from Radames the route the Egyptian army plans to take against them. At first she refuses but, eventually worn down by her father, she agrees. Amonasro hides as Radames enters, believing that if he is successful in battle a second time the Pharaoh will not oppose their marriage. Aida tells him that their only fortune together is in flight to her homeland. Eventually he yields to her, and she casually asks which route the army will take to Ethiopia. As he tells her, Amonasro steps out of the shadows, and Radames is horrified to see what he has done. Both Amonasro and Aida try to persuade him to come with them, but Amneris comes out of the Temple followed by Ramfis. Amonasro tries to stab Amneris but Radames protects her. As Aida and Amonasro escape Radames surrenders to the guards Ramfis has called.
In a hall in the Palace, above the underground Chamber of Justice, Amneris waits alone, torn between her love and a desire for vengeance. She sends for Radames and offers to secure a royal pardon if he will give up Aida, who has made good her escape. Radames refuses and Amneris sends him to his trial. Alone again she hears the charges against him read out and then the sentence of death. As Radames is led up she curses the priests’ cruelty.
In the Temple tomb in which Radames has been sentenced to a live burial he awaits death. The last stone has been put in place when he hears a noise beside him. It is Aida who has stolen into the tomb to die with him. They bid each other farewell while above, in the Temple, priestesses chant and Amneris prays to Isis that Radames’ soul may rest in peace.