Io was a daughter of the river god Inachus in Argolis and a granddaughter of Oceanus. She was the priestess of Hera and a mistress of Zeus, the King of the Gods.
One day, while Io was strolling around, she got noticed by Zeus, who was mesmerized by her beauty and fell in love with her. Io tried to flee, but to no avail, for Zeus spread dark night and prevented her escape.
Soon Hera got suspicious of her husband's absence and she ordered Nyx, the goddess of the night, to retire, so that she could find out about the the illicit affair. In no time, Zeus transformed his lover into a white cow in order to escape his wife's wrath.
However, Hera didn't admit defeat. She compelled Zeus to refuse his illicit relationship with an oath and to bring the animal to her. Zeus was forced to retreat, so Hera took the cow and put her under the supervision of Argos, a giant who was related to her and had hundred eyes scattered all over his body.