High Priest Ramfis announces that Ethiopian forces led by King Amonasro have invaded the Nile valley and marched on Thebes, and that the goddess Isis has already named the supreme commander of Egypt's army. Left alone, young guard captain Radamès dreams of being chosen so he can lay a victor's wreath at the feet of his beloved Aida ('Celeste Aida'). When Radamès is indeed proclaimed general and the court thunders 'Ritorna vincitor!' ('Return victorious!'), Aida is left alone in agony: an Egyptian victory means the enslavement or death of her own father and brothers. After tricking Aida into confessing her love for Radamès, Princess Amneris joins the Pharaoh, High Priest Ramfis, and the populace at the gates of Thebes to celebrate Egypt's victory ('Gloria all'Egitto, ad Iside!'). Radamès is borne in triumph as the famous Aida trumpets blaze. When the Ethiopian captives are led in, Aida spots her father Amonasro among them and rushes into his arms. Whispering fiercely for her not to betray his royal rank, Amonasro tells the Pharaoh that the Ethiopian King died in battle and pleads for mercy on the prisoners. On the eve of her wedding to Radamès, Princess Amneris enters the Temple of Isis by the Nile to pray. Aida arrives veiled for a secret midnight tryst with Radamès; overcome by nostalgia, she bids farewell to her Ethiopian homeland ('O patria mia'). Suddenly her father Amonasro steps out from the shadows: promising Aida that she can have both her throne and Radamès's love, he threatens to curse her if she does not trick Radamès into revealing which route the Egyptian army will take to invade Ethiopia. When Radamès arrives, Aida persuades him to flee with her and asks which path avoids his troops; the moment Radamès names the gorges of Napata, Amonasro springs out proclaiming himself King of Ethiopia! Having refused Princess Amneris's offer to save his life if he renounces Aida, Radamès is condemned by Ramfis and the priests to be buried alive beneath the altar of the god Ptah (Vulcan). As the fatal stone slab seals him in darkness ('La fatal pietra sovra me si chiuse'), Radamès hears a sigh in the shadows: Aida, having learned of his sentence, secretly slipped into the vault ahead of time to die in his arms! While the priestesses chant to Ptah and a heartbroken Amneris weeps above their heads praying for peace ('Pace t'imploro'), the two lovers bid a serene farewell to the earth ('O terra, addio; addio, valle di pianti').