Io was the princess of Argos, who Zeus fell in love with. To try to keep Hera from noticing, he covered the world with a thick blanket of clouds. However, as soon as Hera saw that, she immediately became suspicious. She came down from Mount Olympus and began dispersing the clouds. Zeus did some quick thinking and changed Io's form from a lovely maiden; so, as the clouds dispersed, Hera found Zeus standing next to a white heifer. He then swore that he had never seen the cow before and that it had just sprang right out of the earth. Seeing right through this, Hera faked liking the cow so much that she wanted to have it as a present. As turning such a reasonable request down would have given the whole thing away, Zeus presented her with the cow. She sent the cow away and arranged Argus Panoptes to watch over it. Since Argus had a hundred eyes and could have some of them sleep while keeping others awake, he made for a fine watchman.
Desperate, Zeus sent Hermes to fetch Io. Disguised as a shepherd, Hermes had to employ all his skill as a musician and storyteller to gain Argus' confidence and lull him to sleep. Once asleep, Hermes killed Argus; later, Hera took his eyes and set them into the tail of her favorite bird, the peacock. While Io was now free, Hera sent the mother of all gadflies to sting the still bovine Io. The ghost of Argus pursued her as well. This pushed her towards madness and in her efforts to escape, she wandered the world. During her journeys, she came across Prometheus while chained, who gave her hope. He predicted that although she would have to wander for many years, she would eventually be changed back into human form and would bear a child. He predicted that a descendant of this child would be a great hero and would set him free; his predictions came true. Because of her journeys, many geographical features were named after her, including the Ionian Sea, and the Bosporus (which means ford of the cow). She eventually reached the Nile where Zeus restored her human form. She bore Epaphus and eleven generations later, her descendant Heracles would set Prometheus free.