Clytaemestra comes out of the palace and asks Cassandra to come inside with her. She promises to treat the princess with kindness, but Cassandra does not reply. Clytaemestra repeats her invitation. Cassandra continues to ignore her. Finally Clytaemestra loses her temper and goes inside again, muttering angrily.
There is a moment of silence, then Cassandra steps down from the chariot and cries out in despair that Apollo has destroyed her. The curiosity of the elders is aroused and they encourage her to continue speaking. When Cassandra realizes that she is standing outside the palace of Agamemnon, the House of Atreus, she begins to lament for herself. The elders question her. Cassandra's answers are disjointed and incoherent, but gradually her story becomes clear. She once rejected the advances of Apollo and was punished by him with the gift of prophecy. Now she is able to foretell the future, but Apollo's curse prevents anyone from believing her prophecies. The burden of being unable to communicate her vision is more painful than she can endure.
As she goes on lamenting, Cassandra enters into a prophetic ecstasy. She recounts the whole story of the curse on the House of Atreus, beginning with the feud between Atreus, the father of Agamemnon, and Thyestes, the father of Aegisthus. When Cassandra speaks about past events, the elders are able to understand and recognize that she is telling the truth. But then Apollo's curse takes effect. Cassandra is horrified by a vision of sin and bloodshed. She tries to tell the elders that Clytaemestra is about to murder Agamemnon, but they misunderstand and accuse her of lying. Cassandra realizes that there is no hope of convincing them. She becomes hysterical and foresees her own death and the coming of Orestes to avenge his father. Casting down her prophetic staff and wreath, Cassandra bravely enters the palace to meet her death.
The chorus chants a short lyric on the wickedness of prosperity. Suddenly Agamemnon's voice is heard from inside the palace, screaming that he has been stabbed. The elders mill about in confusion, wondering what course of action to follow. They are about to enter the palace when the doors swing open to reveal the dead bodies of Agamemnon and Cassandra. Clytaemestra is standing triumphantly beside the two corpses.
The result of the short confrontation between Clytaemestra and Cassandra is in striking contrast to Clytaemestra's duel with Agamemnon. By her silence, the Trojan captive shows herself to be a match for the Argive queen.
Cassandra's silence contributes to a feeling of tension, which explodes suddenly after Clytaemestra goes into the palace. Cassandra is a human symbol of Agamemnon's wickedness — he has slain her family, destroyed her home, and violated her in defiance of her sacred oath of chastity. Cassandra's presence underlines the reasons why the gods will allow Agamemnon to be murdered. In a long lyrical speech in which Time seems to be suspended, Cassandra recounts the full circle of sins — past, present, future — that haunt the House of Atreus. She foresees no hope for reconciliation or an end to the curse, for she believes that humanity inevitably is made to suffer at the hands of the gods. At the end of this powerful speech, Cassandra accepts her own fate with dignity. Her last words — "Alas, poor men, their destiny . . ." — generalize her tragic end into the great tragic experience of all humanity, broadening the meaning of the trilogy so that it refers to the most important problems of human religious speculation. Unlike Agamemnon, Cassandra is fully aware of her impending death. At the conclusion of this speech, she enters the palace and dies in silence while Agamemnon's screams echo around her.