Prometheus was the giant who stole fire from the gods and gave it to mortals. As punishment, Zeus shacked him to a rock. Prometheus's own mother was turned into an eagle and was forced to come every day and peck out her own son's liver.
Io was one of Zeus' mortal lovers, whom Zeus turned into a cow when he learned that Hera had caught wind of their affair. Hera asked for the cow, and Zeus reluctantly handed her over. Io escaped from Hera's hundred-eyed watchman, Argus, with the help of Zeus' son Hermes. Hera, upon learning of this, sent a gadfly (represented by the wasp) to chase and torment Io. (Hamilton 81-82)
Io was in a maddened, bestial state when she met Prometheus. Though he was still in chains, Prometheus knew who Io was, and he knew her story. He told her of the future... (78-79)
Prometheus told her that she could only rest when she got to Egypt. There, she would be returned to human form by Zeus, safely out of Hera's sight. (81)
In Egypt, she would give birth to Zeus's child, Epaphus. She would be honored and praised for as long as she lived. From her line would spring the one who would free Prometheus... (81)
...Hercules. Eventually, Prometheus would be freed and would owe it all to Io. (81)