The palace of the Pharaoh in Memphis. Ramfis, the High Priest, is telling Radames, the captain of the Egyptian guard, that the forces of Ethiopia are marching on Thebes and the valley of the Nile. In reply to Radames' question the priest adds that the goddess Isis has named a brave young warrior to lead the Egyptian army against the invaders. Left alone, Radames dreams of the victory and glory that would be his if he were to lead the army. Then his thaughts turn to his beloved Aida who is the young slave of Amneris, the daughter of the Pharaoh. Radames' reverie is interrupted by Amneris. Suspecting that Radames is in love, she questions him guardedly, hoping that she herself is the object of the young man's passion. She does not know - nor does anyone else at court - that her slave Aida is the daughter of Amonasro, the Ethiopian King. Aida enters and Amneris notices the glance that flashes between the other two. Amneris knows where Radames' heart lies. With a show of friendship Amneris deviously asks why Aida is wheeping. The girl replies that she is distressed by the news of war between her country and the Egyptians. Radames suspects that Amneris has found out about the real state of affairs.
The Pharaoh enters with his court and a messanger reports that Ethiopians, under Amonasro, are approaching the city. The Pharaoh announces that Radames has been chosen leader of the army and instructs him to return victorious. Aida expresses her dismay stemming frim her divided feelings of love for Radames and anxiety for her father. She calls on the gods to have pity for her suffering.
At the Temple Radames is blessed as the leader of the army and given the consecrated sword and armour. The ceremony ends with an invocation to mighty Phtha.
A hall in the apartments of Amneris. Amneris is excited in expectation of Radames. The slave girls sing the praises of the victorious warrior. Aida enters and Amneris' doubts and jealousy flare up again. In order to discover the truth, the daughter of the Pharaoh tells her that Radames has been killed in a battle. Aida cries in despair. Amneris in anger exclaims that Radames is alive and that she herself is Aida's rival. Helpless, Aida begs for pity. The sound of festival music is heard outside where the people celebrate the return of the victorious troops. Amneris leaves, having instructed her slave to attend the triumphal ceremonies.
The avenue of liberated Thebes is crowded with people. Radames arrives, celebrated by the exultation of the Egyptians. He is welcomed by the Pharaoh and Amneris. Aida, unable to restrain herself, rushes to her father, but he manages to caution her not to betray him. Nobody should know either his name or his status in Ethiopia. Priests demand that the captives be killed but Radames begs for the mercy for the prisoners. The Pharaoh agrees, though with a stipulation, at the priest's insistence, that all be liberated with the exception of Aida and her father. Finally, without consulting Radames, the Pharaoh announces that Radames will have the hand of Amneris in marriage. Amneris expresses her excitement - her dream come true.
The banks of the Nile. Amneris proceeds to the temple os Isis to ask the goddess to bless her marriage to Radames. Aida waits in secret for her beloved to say her last farewells. Should they not meet again, the Nile would be her grave. Her thoughts turn to the beloved homeland that she will never see again.
Suddenly Amonasro arrives. He knows of the love between his daughter and Radames and that Aida's rival is Amneris. He capitalises on Aida's feelings of love for her homeland and in order to find out the route by which Radames will lead the army against the Ethiopians. Aida refuses to make Radames a traitor. In anger her father curses Aida and tells her she is no longer the daughter of the King but a slave who has betrayed her homeland. Her spirit cannot withstand such torture and she finally consents.
As Radames arrives, Amonasro hides. Radames swears that he loves Aida and is convinced of victory and a happy future. At first Aida listens reluctantly, for she believes that she, the daughter of the King, will ruin the life of her beloved. Their only hope is to flee to Ethiopia. Radames resists, he can't abandon his homeland. Aida reproaches him and finally Radames agrees. In answer to Aida's question as to what path the Egyptians troops will take, he reveals the secret. Amonasro exclaims joyfully - the Ethiopians will be guaranteed victory. But it is not only Amonasro who has heard the secret. Traitor! Amneris cries. Radames is horrified, he has betrayed his homeland. In vain Aida and Amonasro try to calm him and urge him to flee. As Ramfis appears, Radames yields himself to the High Priest. He delays the guards so that Aida and her father can escape.
A hall in the Pharaoh's palace. In un underground chamber the priests determine Radames' fate. Amneris, torn between love and hate, asks that Radames be brought in. She urges him to confess his wrong-doing and oferrs him her love and the throne of Egypt. But he must renounce Aida. Radames refuses: What will my life be without love and honour! - he answers. Then, fool, you must die! - cries Amneris and orders the guards to enter. She collapses as Radames is taken to the judgement room, then tries to overhear what is spoken there. Finally, Ramfis pronounces the sentence: Radames is to be burried alive beneath the temple. Amneris in despair curses the priests for their decision.
Radames is imprisoned under the temple of the God Ra in Memphis. Awaiting death, he thinks of his beloved Aida. Suddenly he hears a quiet voice and sees Aida. She has chosen to die together with Radames. The lovers, in passionate embrace, are ready to die. Nobody will ever separate them. The priests sing in the temple. Amneris mourns at the entrance to the underground cell and asks the gods to pity Radames' soul.