Radamès, a captain in the Egyptian army, learns from the priest Ramfis that the king will designate a general who will lead their forces against the invading Ethiopian armies. Radamès declares his wish to be that general, but in the aria Celeste Aida or Heavenly Aida, his thoughts turn to the woman he loves – the slave girl, Aida.
Amneris, the daughter of the Pharaoh, is in love with Radamès. She approaches him to declare her love. When she is interrupted by the entrance of her slave Aida, she sees how she and Radamès look at each other, which raises her suspicions and turns Aida into her rival for Radamès’ affections.
A messenger arrives to report to the assembled court that the Ethiopians, led by King Amonasro, have crossed the border into Egypt. The pharaoh declares that Radamès will be the general who leads their forces into battle. Aida is torn between her love for Radamès and her love for her country. Unknown to the Egyptians, Aida is actually Amonasro’s daughter and princess of Ethiopia. The crowd urges Radamès and his forces to victory with the words “Guerra” (war) and “Ritorna Vincitor” (return victorious)! Aida, left alone, laments having joined the crowd to wish Radamès victory against her people. Her inner conflict is expressed in a prayer.
The Egyptians have routed their adversaries, and the people await their triumphant return. Amneris dreams of making the victorious general her husband. Aida does not know that Radamès has won the battle, and Amneris tricks her into showing her love for Radamès by telling Aida he has died in battle.
When the truth is revealed, Amneris orders her slave to accompany her to the triumphal feast. Outside the walls of Thebes, jubilant crowds hail the victorious Radamès and his army. The soldiers parade exotic animals, dancers and captured Ethiopian soldiers. As the Ethiopian slaves are presented to the King, Aida recognizes her father, Amonasro, among them.
The Ethiopian King has been captured but has been able to keep his identity a secret to the conquering Egyptians. Radamès asks the pharaoh to release the prisoners as a sign of clemency. Pharaoh agrees, but insists that Aida and her father remain as slaves of Egypt. In gratitude for his victory, Pharaoh gives the hand of his daughter, Amneris, and thus succession to the throne of Egypt, to Radamès.
Late in the evening, Amneris goes to the temple of Isis to ask the goddess to bless her impending wedding to Radamès. Aida enters. She has agreed to meet Radamès. As she waits, she sings of her homeland, so beautiful and yet lost to her forever. Instead of Radamès, her father enters. He has learned of her love for Radamès and tells her that the Ethiopians are ready to attack Egypt once more. He plays on her love of country and convinces her to trick Radamès into revealing the location of a secret path by which the Ethiopians can enter Egypt undetected.
Radamès enters and tells Aida that he will again defeat the Ethiopians, after which he will ask Pharaoh for her hand in marriage as reward. Aida is unconvinced. Afraid of Amneris’ revenge, she instead pleads with Radamès to flee Egypt with her – and she tricks him into telling her the secret path that will allow them to leave undetected. Amonasro overhears Radamès’ betrayal and reveals his true identity to him. When Amneris and the priests come out of the temple, Radamès surrenders to them, as Aida and her father escape.
Radamès is now a prisoner accused of treason. Amonasro has been killed while trying to escape, but Aida has managed to flee to safety. Amneris brings the now disgraced Radamès to her. She begs him to renounce Aida, and if he will, she will intercede with Pharaoh for his life. Radamès refuses Amneris’ offer.
The priests interrogate him, but he refuses to answer them. He is sentenced to a horrible death; he is to be buried alive in the vault of Vulcan’s temple. There, he finds Aida, who has secretly returned. When she learned of his sentence, she hid in the temple to share his fate. As the lovers bid their last farewell, Amneris kneels at the entrance to the vault and prays for her lost love.