ACT I
In a hall of the King's palace, the high priest Ramfis and the warrior Radames discuss Ethiopia's imminent invasion of Egypt. Ramfis leaves to deliver to the King the name of the general whom the goddess Isis has chosen to lead the Egyptians. Alone, Radames dreams of being selected and returning to Egypt as a glorious conqueror, dedicating his victory to the woman he loves, the Ethiopian slave, Aida. Amneris encounters him and asks why he appears in such rapture. He skirts her probing questions, but when Aida enters, his reaction focuses Amneris's suspicions. The King arrives and listens to a messenger explain that Ethiopian troops have attacked Egypt's borders. The Ethiopians are led by their king, Amonasro—Aida's father. The King announces whom Isis chose to lead their own army: Radames. The politicians, officers, and priests call for war and for the annihilation of the Ethiopians. Aida is stricken to find herself joining in the chorus. Torn between her devotion to her father and her love for Radames, she prays to the gods to end her misery.
At the Temple of Ptah, the priests and priestesses enact a ritual after which Radames receives a sacred sword in token of his future victory.
ACT II
Under Radames' leadership, the Egyptian troops have been victorious. Amneris is being prepared for the celebration. Wishing to test her suspicion, Amneris tells Aida that Radames was killed in battle. Aida's anguished reaction fuels Amneris's suspicion. When Amneris then admits that Radames is really alive, Aida can no longer conceal her feelings. Amneris declares herself a rival, prompting Aida to ask for pity. Amneris vows revenge and ill-fortune on Aida.
In a triumphal parade, Radames and his soldiers march into the city. When the captives are brought before the King, Aida is shocked to recognize her father in the throng. The King presses the man for his identity. He admits to being Aida's father, but says that King Amonasro perished in battle. He asks the King to show mercy on his people. Won over by the nobility and weakened state of the Ethiopians, the Egyptian people ask the King to show clemency. Against his priests' advice, the King spares the prisoners' lives, keeping Aida's father and Aida as hostages. He also publicly awards Radames his daughter's hand in marriage.
ACT III
In preparation for the wedding, Amneris and Ramfis come to worship in the Temple of Isis. Aida is waiting to meet Radames. Amonasro appears and suggests that there is yet a way to live in bliss with Radames in the valleys of her childhood. Ethiopian forces have regrouped and wait to attack again; they need only to know where the Egyptian army will move. Amonasro wants Aida to discover the path of Egyptian advance. Aida is appalled when he hints that she could secure the information from Radames. But Amonasro squashes her resistance by taunting her with visions of her dead mother and the destruction of Ethiopia. When he hears Radames approach, he conceals himself. Radames swears eternal love to a skeptical Aida and insists that once he defeats the Ethiopians, he can claim his own prize: Aida; then they can be happy. Aida entreats him to flee with her outside the country. In the wilds, she says, they can be free. As they begin their departure, Aida asks him about Egyptian troop movements, ostensibly so that they can be sure of evading them. Radames reveals the information. Amonasro now emerges from his hiding place and reveals himself as the King of Ethiopia, promising Radames the hand of Aida for his help. Just then, Amneris and Ramfis emerge from the temple. Amonasro attempts to kill Amneris, but is stopped by Radames. Radames helps Amonasro and Aida flee, but remains behind to face the religious authorities.
ACT IV
Torn between love and hatred for Radames, Amneris at last resolves to save him from the priests. If he will deny the charge of treason, she tells him, she will intercede with her father and save his life. But Radames refuses to offer a plea to the priests. She offers him everything it is in her power to give him if he will plead innocent and renounce Aida, but he refuses. She denounces him But after the guards lead him away, she is desolate, and curses her jealous fury, as well as the priests who follow their law unfeelingly. In the subterranean hall of justice, Radames is charged with treason and three times is asked to defend himself. Three times he refuses to speak on his own behalf. The priests sentence him to be entombed alive. Hearing the verdict, Amneris is enraged at the hypocrisy of the priests, and curses them. But they are unmoved and declare that their sentence will be carried out.
Sealed within his tomb, Radames discovers Aida hidden there. Knowing his fate, she has choosen to join him, finally, in death. Sounds of the priests' ritual dances drift through the walls as the two lovers sing their devotion to one another. They bid farewell as they wait for death, together at last. Outside, Amneris—clad in mourning attire— prays to the goddess Isis to grant Radames eternal rest.