The definition of the "Call" phase in Booker's Seven Basic Plots is that "something has gone wrong" and only the hero can put it right. At the beginning of Prometheus Bound, something has definitely gone wrong: the hero, Prometheus, the guy who gave fire to man, has been chained to a rock in the Scythian wilderness. But there's a twist: Prometheus's quest doesn't involve him going anywhere. It's all about the battle of wills raging in his own mind.
Gee, that sounds fun to watch.
What's a hero in a quest without some friends to accompany him on his adventure? Right on cue after Hephaestus, Power, and Violence check out, the Chorus of daughters of Oceanus show up. They heard him being chained up, and wanted to make sure he was okay.
Yeah, the hero's chained to a rock. Fun quest story, right? Part of the "Journey" stage of a typical quest story involves "Temptations," where some external force tries to tempt the hero into departing from his quest.
In Prometheus Bound, this temptation comes in the form of Oceanus, who tries to get Prometheus to let him speak to Zeus on his behalf. But Prometheus's mission is to defy Zeus, so Oceanus is just something trying to tempt him away.
Even though Io may not look like she's in much of a position to help at the moment (those pesky hooves), she's going to be a big help in the future: she'll give birth to a line of important descendants, one of whom, one day, will free Prometheus from his imprisonment on the rock.
Finally, Hermes comes down from Zeus and demands to know who is destined to kick Zeus off his throne. If Prometheus doesn't hand over this information, he'll be in serious trouble. But will he be able to hold out against Hermes's terrifying threats? (Hint: yes.)
Okay, so under ordinary circumstances, an ending where the hero gets kicked down to the utter depths of subterranean darkness wouldn't be something to celebrate. In this play, though, it simply shows that Prometheus had the courage to stand up to the most powerful god in the universe. And that makes this a happy ending in our book.