Io was a Naiad nymph in Argos. Zeus fell in love with Io and lusted after her. Io rejected Zeus advances until the oracles caused her own father to drive her out into the field of Lerna. Zeus pursued Io, covered her with clouds to hide her from the eyes of his wife (Hera), and seduce her.who was seduced by Zeus, who changed her into a heifer to escape detection. Hera however suspected a trick and requested Zeus give her the cow. As it would be odd to refuse such a simple request Zeus consented.
Hera tethered Io to the olive-tree in the temenos of her cult-site, Hera then placed Argus in the charge of many-eyed Argus Panoptes to keep her separated from Zeus. Zeus commanded Hermes to kill Argus. Hermes lulled all hundred eyes to sleep then killed the giant with an adamantine sword. Before Zeus could rescue Io Hera set a gadfly to sting Io causing her to go mad and wander across Europe and Asia, go across the Ionian Sea. During her travels, she came across Prometheus while chained, and everyday a giant eagle, fed on his liver. Despite his agony, he gave Io hope. He predicted that she would have to wander for many years, but she would eventually be changed back into human form and would bear a child and become the ancestress of the greatest of all heroes.
- in some versions Hera only permitted Zeus to change Io back if Zeus agreed never to see Io or her child again. This is however doubtful since after giving birth to Zeus' son, she would later bare Zeus a daughter.
- Eleven generations after Zeus had sex with Io, Prometheus' prediction came to pass Io's decendant Herakles would be born.