Orestes and the tutor have plans to avenge Agamemnon's death, Orestes' father. Orestes hasn't seen his sister in years because Electra didn't want him growing up with a mother like Clytemnestra.
Clytemnestra has a prophetic nightmare about Agamemnon. She saw him return to life holding the scepter from Aegisthus. She was terrified he would somehow come back, so she told Chrysothemis (her daughter) to offer some offerings to her father's tomb.
As you can see, I am not dead!!
A messenger announces Orestes is dead because of an "accident" in the Olympic games. This news, of course, upsets Electra, but couldn't say the same for her mother, Clytemnestra. She feels relieved that Orestes is now dead.
Thank lord Zeus.
Orestes reveals himself to Electra, filling her with joy. He is not dead as the messenger said, it was part of their plan all along.
Clytemnestra is then alone in the palace with no one in sight. Orestes, the tutor, and Pylades decide it was a perfect time to attack and kill her.
Aegisthus discovers Orestes is alive after all; he wasn't very happy with this news. Orestes is leading him to a trap and some shocking news. He then is lead into the palace and is killed by Orestes and Pyaldes.