Agamemmon returns to Argos from the Siege of Troy, bringing, as a prisoner and trophy of war and victory, Cassandra, the beautiful daughter of King Priam of Troy. Clytemnestra, the wife of Agamemnon, has a lover, Aegisthus, who fans the flames of jealousy aroused in Clytemnestra by the arrival of Cassandra, with whom Agamemnon is very much infatuated.