Act I
Egypt, during the reign of the pharaohs. At the royal palace in Memphis, the high priest Ramfis tells the warrior Radamès that Ethiopia is preparing another attack against Egypt. Radamès hopes to command his army. He is in love with Aida, the Ethiopian slave of Princess Amneris, the king’s daughter, and he believes that victory in the war would enable him to free her and marry her. But Amneris loves Radamès, and when the three meet, she jealously senses his feelings for Aida. A messenger tells the king of Egypt and the assembled priests and soldiers that the Ethiopians are advancing. The king names Radamès to lead the army, and all join in a patriotic anthem. Left alone, Aida is torn between her love for Radamès and loyalty to her native country, where her father, Amonasro, is king. She prays to the gods for mercy.
In the temple of Vulcan, the priests consecrate Radamès to the service of the god. Ramfis orders him to protect the homeland.
http://www.metopera.org/metopera/season/synopsis/aida