Radames, 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. Radames 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 Radames. She approaches him to declare her love. When she is interrupted by the entrance of her slave Aida, she sees how she and Radames look at each other, which raises her suspicions and turns Aida into her rival for Radames’ 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 Radames will be the general who leads their forces into battle. Aida is torn between her love for Radames and her love for her country. Unknown to the Egyptians, Aida is actually Amonasro’s daughter and princess of Ethiopia. The crowd urges Radames 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 Radames 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 Radames has won the battle, and Amneris tricks her into showing her love for Radames 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 Radames 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. Radames 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 Radames.