Egypt is at war with Ethiopia. Radamès is chosen to command the army. Aida is torn: he will go to fight against her father and her people. Radamès wins the war and returns in triumph. The Pharaoh grants him any wish. Among the prisoners is Amonasro, Aida's father — who hides his identity as king.

At night, by the Nile. Amonasro persuades Aida to extract from Radamès the secret of the Egyptian army's route — information that would allow the Ethiopians to ambush them. Aida, torn apart, does it. Radamès reveals the military plans. Amonasro exults — he had been listening. Radamès understands he has been used. He is arrested as a traitor.

Amneris begs Radamès to defend himself, to save himself. All he has to do is give up Aida. He refuses. He is condemned to be walled alive in a tomb beneath the temple. When the stone falls, Radamès hears a breath: Aida has hidden inside to die with him. They embrace in the darkness while above them Amneris weeps.