Orestes is back in town with Pylades and the old slave. They recount, for the audience's benefit, the past action: Agamemnon was killed and Orestes was smuggled out of town by his sister Electra and the old slave.
Orestes declares that he's ready for action; he sends the old slave to the palace with a faulty story about his own death, and heads to Agamemnon's grave to pay his respects to Dad.
Orestes comes on stage with Pylades; he's carrying a small urn, which supposedly hold his (Orestes's) ashes. When he realizes that he's conversing with Electra, he reveals his true identity. Together, they plot to kill the royal couple.
Orestes goes inside with Pylades and kills Clytemnestra.
He comes outside with the covered body and converses with Aegisthus. During the conversation, Orestes reveals his identity.
Orestes ushers the Aegisthus inside, where he plans to murder the King.