The play begins in the barren wasteland of Scythia. Three gods—Hephaestus, Power, and Violence—are leading a fourth, Prometheus, who is their prisoner. Hephaestus chains Prometheus to a rock. Hey, dude, he's only following orders. Specifically, Zeus's orders.
After Hephaestus, Power, and Violence leave, Prometheus treats us to a little story about why he's all tied up: Zeus is punishing him for taking fire from the gods and giving it to mortal humans. Just then, some company arrives for poor old Prometheus: it's the Chorus of the daughters of Oceanus, who just happened to be in the neighborhood and hear him being chained. Oh, great! A little musical entertainment.
Not so much. When the Chorus asks Prometheus how he got himself into this fix, Prometheus fills them in on the backstory of how his friendship with Zeus went sour, leading up to the moment when he stole fire and gave it to humans.
Then, Prometheus invites the Chorus to gather at the foot of his cliff to listen to his prophecy about the future. Just as they're heading down, their dad shows up. Oceanus tells Prometheus that he'll talk to Zeus and get him out of this mess. Great! Shortest play ever.
Again, not so much. Prometheus refuses.
Then Prometheus fills in a bit more detail about his relationship with humans, revealing that he taught them all the technological knowledge they have. Worrying that he will only make Zeus angrier, the Chorus tells Prometheus to stop bragging. He refuses, adding that Zeus won't rule forever anyway. Prometheus won't explain his prophecy, because the knowledge is the only leverage he has.
And then a cow charges onto the scene! But not just any cow: it's the mortal girl Io, who has a story of her own to tell. See, Io used to have strange dreams telling her that Zeus, king of the gods, was in love with her. Eventually, her father consulted an oracle, who told him to send Io out into the wilderness for Zeus. Pretty soon: cow. When Prometheus says that Io's troubles have just begun, Io contemplates suicide. She changes her mind when she learns that her sufferings will last for hundreds of years—but not forever.
Prometheus's suffering won't last forever, either: eventually Zeus is going to fall from power, but even before that he'll be set free by one of Io's descendants. Whaa? Io doesn't have any descendants. Oh, says Prometheus, but she will: she's going to get it on with Zeus. (Oh, and he'll transform her back into human form—we hope before the sexytimes.)
Io, understandably, is stressed out by this news, so she starts dancing offstage. Like you do. And then someone else shows up: it's Hermes, the messenger-god. He insists that Prometheus spill the details on who's going to knock Zeus off his throne, but Prometheus send him packing (and calls him a few fun names along the way).
After Hermes leaves, the Chorus keeps trying to convince Prometheus to just say he's sorry already, but he can't keep his mouth shut about what a tool he thinks Zeus is. We bet you can guess what comes next: lightning flashes, a giant cavern opens up in the earth, and Prometheus disappears into the void. Dun dun dun. To be continued!