As he is
wont to do,
Zeus fell in love with a
mortal girl. He
seduced Io, the
beautiful daughter of Inachus, who was the
legendary founder of
Argos. To cover his tracks and avoid his wife
Hera's suspicious eye, Zeus covered the
world in a
blanket of
clouds. All Zeus managed to do, however, is draw attention to himself
frolicking with Io in a field.
Hera came storming out of Mount Olympus, clearing away the clouds, looking for Zeus. Knowing the jig was up, Zeus changed Io from a beautiful maiden to a black and white heifer. Hera happened upon the god and the cow shortly after, looking for an explanation. Zeus swore up and down that he hadn't seen the heifer before, and that it must have just sprung up from the earth.
Hera, who knew better than anyone when Zeus was lying badly, made a wise and shrewd move at this point. She commented on what a fine heifer it was, and asked if she could have it as a present. Knowing he couldn't deny her this without arousing even more suspicion, Zeus allowed her to keep the cow. She isolated the cow and had the giant Argus to keep a hundred or so eyes on her.
Desperate for Io's return, Zeus sent Hermes to rescue Io. Hermes disguised himself as a shepherd and lulled the giant to sleep with story and song. Once Argus was asleep, Hermes killed him and led Io away. For his dedicated (though negligently somnolent) service, Hera took Argus' hundred eyes and set them into the tail feathers of the peacock, her favorite bird.
Enraged, Hera sent a savage gadfly to sting and madden the heifer. The gadfly pushed Io to the edge of sanity, sending her fleeing around the world to escape it. In her travels, she encountered Prometheus, chained to a rock, who prophesied that one day, her descendant would come and set him free.
This prophecy gave Io hope throughout her wanderings, and eased the pain of the gadfly. Eventually, she came to the Nile River in Egypt, where Zeus set her free. Eleven generations after her child Epaphus was born, her descendant Hercules liberated Prometheus. Because of her travels, Io's name is left on a few earthly bodies of water, including the Ionian Sea and the Bosporus Strait (Bosporus translates roughly as "bovine ford"). One of the moons of Jupiter is also named for Io.