From the invocation of Agamemnon's spirit to the change of scene from the king's tomb to just outside the door of Clytaemestra's palace. (Lines 315-652):
In a lyrical and intense sequence, Electra and Orestes, and to a lesser extent the Chorus, call on the spirit of Agamemnon to aid them. They pray to Zeus, Earth, and the gods of the dead. They speak of the wrongs committed by Clytaemestra and the shameful end of their great father. They pray for aid and strength so that they will not become soft and fail to carry out the deed.
The Chorus praises the words of Electra and Orestes spurs them on to action. Orestes asks why Clytaemestra decided to send offerings to Agamemnon's tomb. The Chorus tells Orestes that the queen had a dream that she was nursing a serpent. The snake drew blood along with mother's milk. Orestes hopes that the snake symbolizes him. Electra will go inside and keep watch on the house to make sure nothing gets in the way of their plot. The slave women will keep Orestes' return a secret. Orestes and Pylades will go to the gate of the palace, disguised as foreigners. Because of the Greek view of hospitality, they will not be turned away. Once inside, Pylades and Orestes will kill Aegisthus first. Orestes and Pylades exit, and Electra exits separately.
The Chorus sings of the sins of evil women and men. Time and time again, the gods have shown that they punish wrongdoers. The Chorus describes "daring" as an inextricable part of their sins. These villains dared to defy the gods or the laws of morality. For the sake of ambition or lust they put themselves above the laws of gods and men. The Chorus says that there are clear lessons to be learned from the fate of these sinners. Their punishment is a sign that the gods watch and judge. In Argos, the Chorus says, those in sin have yet to be punished, but at long last Destiny and the gods will bring about their just end.
The invocation of Agamemnon is one of the most powerful scenes of the play. The language is intense, full of violence and passion, and through this language Aeschylus conjures the sense of tremendous powers at work in the events to come. The divine plan and destiny is an important theme of the trilogy. Remember that Apollo himself has given the order for Orestes to murder his mother. Also at work is the theme of history and memory. The actions of Orestes are spurred from above, but they are also spurred from behind: in the past, he has the whole bloody history of his family and the Curse on the House of Atreus pushing him forward. Several times during the invocation, the Curse comes up either explicitly or implicitly. There is a strong sense of the legacy of violence that has been left to these children; Electra describes herself and Orestes as "savage born from the savage mother" (l. 422), and the Chorus speaks explicitly of the terrible internal violence that has plagued the house of Atreus.
Two themes come together in the Curse: the theme of history and the theme of violence's self-perpetuating nature. Behind Agamemnon's death is the story of the Trojan War and before that, the bloody legacy of his father. (See the ClassicNote on Agamemnon or this ClassicNote's Short Summary for the story of the Curse on the House of Atreus.) Remember that Aegisthus was Thyestes' only surviving son. His brothers were killed by Agamemnon's father through a scheme that was both barbaric and wickedly creative. Aegisthus, in turn, killed Agamemnon with Clytaemestra's help. Now, Orestes will kill him. Violence is a cycle that seemingly has no end. The force of this cycle propels Orestes toward matricide.
But that does not mean that matricide is easy or pleasant. When Orestes and Electra call on their father to remember their suffering and the wrongs committed against him, the invocation seems to be as much for them as it is for him. They are working themselves into a frenzy, making themselves bloodthirsty and savage enough to murder their own mother. This frenzy comes with the theme of memory and history. Memory gives the past the power to work on in the present; through the invocation, Orestes and Electra are making themselves relive the horrible events of the past. They are manipulating memory, (leaving out Iphigeneia's death, for instance) so that they will be able to properly avenge their father, as ordered by Apollo, and protect their lives and their rightful inheritance.
In Electra's final prayer to Agamemnon, she brings up one of the important symbols of the trilogy. She likens her brother and herself to the corks of a net, which keep the fibers from sinking. In this way, though their father is dead, he will live on if his children survive. They will hold up his bloodline for him. The impetus to restore order, to set the world right, is another important theme of the trilogy. Part of restoring order is putting the rightful heir on the throne and continuing the family line. But Electra's use of metaphor shows how complex symbolism in the Oresteia can be. The corks of the net are a metaphor for the family's survival, but the net and water imagery recall Agamemnon's murder. Remember that to kill him, Clytaemestra ensnared the king with a piece of cloth while he was taking a bath. Even in an image of survival, there is no escape from images of violence. Survival will not come with clean hands for the siblings; more brutality and treachery will be needed.
But still, Aeschylus shows faith in the ultimate triumph of justice. His universe is not as chaotic or malicious as Euripides'; the words of the Chorus show that the gods watch and judge justly. Although solutions are not simple and often unmerciful, there is a sense that in the end all comes to right. Clytaemestra's death has to happen; the murder will be further proof that there is justice in the world.