The basic story picks up a couple of generations into a miasmic pollution or inherited curse that began when Tantalus fed his son Pelops to the gods (I'm not going to worry about spoilers in a 2,000-year-old play). Later, Thyestes, the son of Pelops, ends up eating his own children after his brother Atreus chopped them up and cooked them. So Atreus' two kids are Menelaus, who married and was later cuckolded by Helen (later "of Troy"), and Agamemnon, who led the Greeks against the Trojans in an attempt to get her back. Anyway, you know some bad shit is going to happen here at Argos, Agamemnon's town. When he comes back his wife Clytemnestra, who has a new dude, kills him and Cassandra, the enslaved Trojan princess. Their other daughter Electra largely dithers, while the son Orestes, who has been in exile, returns and kills his mother. The furies haunt him, and Athena absolves him, turning the furies, who prosecuted blood murder and specifically matricide, into the Eumenides, or the titular "Pretties," and subdue the old, pre-Hellenic matriarchal forces with the paternal power of Zeus.