Through Orestes's conversation with the old slave, we get some back-story on Electra: when Agamemnon was murdered, she smuggled Orestes out of town for his own safety.
Electra's comes on-stage with a grief-filled monologue. Her life is terrible; she's abused by her mother, and all she can do is hope and pray that her brother Orestes will soon return to Mycenae, which he's been promising for some time now.
She converses with the Chorus. They think she should be more practical and stop obsessing over the past.
Electra argues with Chrysothemis. Electra can't believe her sister plays nice with the people who murdered their father. Chrysothemis reveals that Aegisthus is fed up with Electra's behavior and isn't going to put up with it much longer.
Electra instructs her sister to pay her respects to Agamemnon's grave.
Electra argues with Clytemnestra about Agamemnon's murder; she gives her mom no moral wiggle room.
Electra grieves when she hears the (fake) news that Orestes is dead.
She asks Chrysothemis to help her kill the royal couple, and then resolves to do it on her own.
Electra is reunited with her brother Orestes and rejoices. She encourages him to kill the Queen and listens in (while cheering him on) as he does.