The beauty of the cow mesmerizes Hera but also arouses her suspicions. To completely dispel Heras doubts, Zeus gives her the cow. Hera assigns a shepherd named Argus to protect the cow. Arguss unique trait is that he never sleeps and can always keep an eye on the cow. Argus grazes the cow in the meadow during the day and keeps her in a closed stable at night.

Unable to bear Ios suffering, Zeus sends his son Hermes to rescue her. Hermes devises a plan to put Argus to sleep and save Io and seeks the help of the Sleep God, Hypnos. Hypnos has poppy flowers that induce sleep. Hermes takes some of these flowers from Hypnos and goes to Argus. Disguised as a shepherd, Hermes fills his flute with poppy flowers and plays it to Argus. Argus falls asleep due to the sound of the flute and the scent of the flowers, and Hermes rescues Io. However, Hera doesnt give up on Io and sends a large gadfly after her. Io, tormented by the flys bites, starts running.

She first heads towards the sea in western Greece, then crosses into Thrace, and eventually crosses from Europe to Asia through the Bosphorus. Io runs relentlessly through the highlands of Anatolia and finally reaches Egypt. Zeus kills the gadfly chasing Io on the banks of the Nile and restores her to her original form. Io has a daughter by Zeus and names her Keroessa. Keroessa is the mother of Byzas, the founder of the city of Istanbul. When Byzas establishes the city, he names the Bosphorus after his mother, which means ""the Cows Ford.""