In ancient Egypt, at the royal palace in Memphis, Ramfis, Egypt’s high priest informs Radames, the warrior that Ethiopia is plotting another attack against Egypt. Radames hopes to be the leader of the army where he can achieve victory and rescue Aida, his Ethiopian lover. Aida is imprisoned and serves as slave to the Egyptian Princess, Amneris, daughter of the King who happens to be in love with Radames. A messenger informs the King of Egypt that the Ethiopians have already made their way into Thebes and names Radames the leader of the army. Left alone, Aida torn between her love for Radamès and her loyalty to her native country, where her father, Amonasro, is king.
After the defeat of Ethiopia, Amneris waits for the victorious return of Radames. As Aida approaches the Princess, she attempts to confirm her suspicions of Aida and Radames by telling her that Radames has died during the battle. Aida’s reactions leaves no doubt that she loves Radamès. Amneris, certain she will be victorious over her rival, leaves for the triumphal procession. At the city gates the king and Amneris observe the celebrations and crown Radamès. Captured Ethiopians are led in. Among them is Amonasro, Aida’s father, who makes her promise to not reveal his identity as king. Radamès is impressed by Amonasro’s eloquent plea for mercy and asks for the death sentence on the prisoners to be overruled and for them to be freed. The king grants his request but keeps Amonasro in custody. The king declares Radames as his successor and future husband of Princess Amneris.
On the eve of Amneris’s wedding, Ramfis and Amneris enter a temple on the banks of the Nile to pray. Aida, who is waiting for Radamès, is lost in thoughts of her homeland. Amonasro suddenly appears. Invoking Aida’s sense of duty, he makes her agree to find out from Radamès which route the Egyptian army will take to invade Ethiopia. Amonasro hides as Radamès arrives and assures Aida of his love. They dream about their future life together, and Radamès agrees to run away with her. Aida asks him about his army’s route, and just as he reveals the secret, Amonasro emerges from his hiding place. When he realizes that Amonasro is the Ethiopian king, Radamès is horrified by what he has done. While Aida and Amonasro try to calm him, Ramfis and Amneris step out of the temple. Father and daughter are able to escape, but Radamès surrenders to the priests.
Radamès awaits trial as a traitor, believing Aida to be dead. Even after he learns that she has survived, he rejects an offer by Amneris to save him if he renounces Aida. When he is brought before the priests, he refuses to answer their accusations and is condemned to be buried alive. Amneris begs for mercy, but the judges will not change their verdict.
Aida has hidden in the vault to share Radamès’s fate. They express their love for the last time while Amneris, in the temple above, prays for Radamès’s soul.