When the story begins, Zeus looks down from Olympus and sees a smokin' hot nymph named Io. Next thing you know, he's got her surrounded by clouds, and they're getting busy. Hera (Zeus's wife) gets suspicious, so she zooms down to catch her husband in the act.
Sneaky Zeus thinks fast and turns Io into a white cow to hide her from his wife. The Queen of the Gods is a pretty smart cookie, though, and she asks for the cow as a present. Zeus can't say no, so the cow-formerly-know-as-Io ends up being guarded by Hera's henchman, Argus Panoptes.
Not one to let his wife punk him down, Zeus sends Hermes to put Argus to sleep and chop off his head. Hera isn't amused, so she sends a gadfly to sting Io wherever she roams. After a seriously crappy tour of the world, Io ends up in Egypt, where Zeus finally has the decency to turn her back into a nymph and get Hera to shoo the gadfly away.
Next thing you know, Io is worshipped all over Egypt as the goddess, Isis.
The Less Short Story
The river god, Inachus, is mad worried about his daughter, Io, the naiad. She hasn't come by in a while—no calls, texts, nothing.
(Note: In some versions of this story, Inachus is the first king of Argos and Io is his daughter. Sometimes Io is said to be a priestess of Hera, too. So many options.)
It turns out Inachus' lovely daughter has had the very bad luck to catch the attention of Zeus, the king of the gods.
When Zeus first sees Io, he zooms down from Olympus and tries to seduce her all smooth like in the woods.
Io doesn't buy it. She says, "No way, Jose," and keeps on walking. Smart girl.
Zeus doesn't know how to take no for an answer, so he covers the woods in mist and cloud.
The story differs here depending on which version you read. Some say Zeus appears out of the cloudiness and seduces Io. Other versions say that he forces himself on her. Either way, not cool.
Up on Olympus, Hera looks down, sees all the cloudiness, and gets way suspicious.
The queen of the gods zips down to Earth to see if her husband is up to no good—which is usually the case.
Zeus sees her coming, though, and does some quick thinking: he turns Io into a white cow (or heifer).
Yep. When Hera gets there, she asks Zeus what he's up to, and he's like, "Oh, just hanging out with this white heifer in these woods that I made cloudy for no particular reason."
Hera isn't as dumb as Zeus thinks. In fact, she's not dumb at all.
"Cool," says Hera. "I just love white heifers. Why don't you show me how much you love me and give it to me as a gift?"
Zeus is totally cornered. If he makes a big deal about giving up the cow, Hera will know something's up.
"Fine," says Zeus. "She's all yours." [Insert a very sad "moooooooooooooooooooo" from Io.]
More than a little suspicious of this heifer, Hera gets the giant Argus Panoptes to watch over her.
She figures Argus is a good pick for watchman because his body is totally covered with eyes. Argus lets Io graze all day, but at night she gets locked up. So yeah, her life is pretty terrible at this point.
One day, Io is doing some grazing by the river of her dad, Inachus. She scratches out her name in the sand with a hoof. (And you thought cows couldn't spell.)
Inachus is majorly depressed when he figures out that the white heifer is his daughter, but he can't do anything to help her for fear of Hera.
Finally, Zeus steps in to help out. The king of the gods sends Hermes to spring her from captivity.
Hermes flies down to the grove where Argus is guarding Io and starts playing some music. (Some versions say he plays a syrinx or panpipes, while others say he plays a lyre.)
The music makes Argus totally relaxed, and the many-eyed giant shuts all his peepers.
As soon as Argus' hundred eyes are closed, Hermes lops off his head. Harsh.
Io, who can now go free, runs stampeding off through the fields.
Hera is pretty upset about what happened to her henchman, so she puts his eyes on her favorite bird, the peacock. So that's why they look like that.
The queen of the gods isn't about to let Io graze in peace, though. So she sends a gadfly to sting the white heifer wherever she roams. (In some versions, Hera sends the Furies to torment Io, which is even worse.)
Io roams all over the place, being stung the entire time.
In some versions of the story, Io wanders by Prometheus, who's chained to a rock in the Caucasus Mountains. They end up chatting for a while about how Zeus kind of sucks before Io pushes on. (To find out why Prometheus is anti-Zeus click here.)
Eventually, Io ends up in Egypt, where she's thinks to herself, "enough is enough." She lifts up her nose to the heavens and lets out a seriously sad moo.
Zeus hears her and begs his wife to let up on Io. He swears he won't have anything more to do with her if Hera will just stop torturing her.
Hera agrees and calls off the gadfly, and Zeus changes Io back into a beautiful nymph.
Io decides Greece is a terrible place to live—it didn't treat her so well, after all—so she hangs out in Egypt. According to some, she eventually becomes widely worshipped in Egypt as the goddess, Isis. Ah, success.
Not long after, Io has Zeus's son, who apparently she's been pregnant with this whole time.
The kid's name is Epaphus, and he ends up being the first king of Memphis. (Egypt not Tennessee.)
Eventually, Io's descendants through Epaphus head back to Greece. Cadmus founds Thebes and Danaus heads back to Argos with his fifty daughters.