This story is written by both Aeschylus and Ovid.
One day, while Prometheus was serving his punishment for his kindness toward man, he had a strange visitor. The visitor looked like a cow but had the voice of a girl who seemed mad with misery. As the visitor, Io, approached, Prometheus recognized the visitor. He knew her story and he spoke her name.
When Io found out that the person chained to a rock with unbreakable adamanite chains was Prometheus, they started to talk to each other. Prometheus told Io his story and Io told him why she, once a princess and a happy girl, was turned into a cow by Zeus.
After Zeus fell in love with Io, Hera grew jealous. When Hera found out about their relationship, Zeus tried to hide himself and Io by wrapping a cloud around the earth. He hoped that this cloud would be so thick that it would leave the place in darkness.
But Hera knew why this odd occurrence was happening. As Hera called off the cloud, Zeus quickly transformed Io into a cow. He claimed that the cow had been born from the earth. Hera did not believe this and she made Zeus give the cow to her. Zeus reluctantly handed Io, in the form of a cow, to Hera, who had a plan to keep her away from Zeus.
She handed Io into the charge of Argus. Since he had a hundred eyes, he was able to sleep with some of his eyes and guard Io with the rest. Zeus, unable to withstand Io's misery told Hermes to kill Argus. Hermes, knowing of Argus' abilities, bored the Argus with stories and music. When Argus' eyes finally all fell asleep, Hermes killed him but not before Hera took his eyes and placed them on her favorite bird, the peacock.
Io thought she was free but once again, Hera sent to gad-fly to drive Io into madness.
Prometheus tried to comfort Io, but he could only tell her to look to the future. After speaking to Prometheus, Io wandered on. One part of a sea which she wandered through was named after her (Ionian Sea) but real consolation came when she reached the Nile. There, Zeus returned her to human form. They would have a son, Epaphus, and live happily and in honor. Io's descendant in the future would be Hercules. It is he that Prometheus would later owe his freedom to.