Zeus found Io wandering alone and aimlessly soon thereafter. Nor did it take Hera long to locate her errant priestess. Seeing his wife coming, Zeus turned Io into the first cow.
Hera, who herself would be known for white arms and bovine eyes, took a fancy to the beast and with more than a drop of suspicion in her veins, insisted her husband give her the animal for a gift.
What else could Zeus do? The hapless philanderer wasn't so much concerned with being caught in a lie as his precarious marital harmony, so he acceded to his wife's wishes.
Poor Io! The unwilling partner-turned-cow was put under the protection of Argus of the hundred eyes.
With so many eyes, he could never be caught entirely asleep. Io was a prisoner. Other business consumed Zeus' time until his crafty son Hermes came to his attention. With a flash of inspiration, Zeus realized Hermes could figure out a way to distract Argus and free poor cow-formed Io.
First Hermes tried music to lull the monster to sleep. He played on the pipes his son, Pan, had created from the reedy remains of his lost love, the nymph Syrinx. Many of the eyes closed, but -- unfortunately for Argus -- not all simultaneously. So Hermes devised a more devastating plan. The details are murky. Whether Hermes hurled a rock or sliced with a blade, he effectively dissociated the guard from his head and eyes.
Io was freed, but still a cow and not quite safe, because Hera set a gadfly to pester her. Io roamed mile after mile, year after year until she finally came to term. When she reached Egypt, she gave birth. Then hera, the childbirth goddess and Io's former nemesis, released her from her torments.
Meanwhile Hera, loathe to waste such useful eyes as those belonging to the severed head of Argos, took them and inserted them into the tail feathers of her favorite bird, the peacock.
Story of Io by Aeschylus
The story from Aeschylus' Prometheus Bound
But the high hand
Of Zeus bear hardly on the rein of fate.
And, instantly-even in a moment-mind
And body suffered strange distortion. Horned
Even as ye see me now, and with sharp bite
Of gadfly pricked, with high-flung skip, stark-mad,
I bounded, galloping headlong on, until
I came to the sweet and of the stream
Kerchneian, hard by Lerna's spring. And thither
Argus, the giant herdsman, fierce and fell
As a strong wine unmixed, with hateful cast
Of all his cunning eyes upon the trail,
Gave chase and tracked me down. And there he perished
By violent and sudden doom surprised.
But I with darting sting-the scorpion whip
Of angry Gods-am lashed from land to land.