Medusa was a mortal woman whom Athena changed into a Gorgon as punishment for desecrating her temple by sleeping with Poseidon there. Some say that Poseidon raped her, while others say that she willingly had intercourse with him in Athena's sanctuary. Nonetheless, when Athena became aware of these activities going on in her temple, she became enraged. It may have been jealousy that provoked Athena to turn Medusa from a beautiful woman into the gorgon, for Medusa was reputed to be pulchritudinous, and her hair was particularly splendid. When Athena came upon Medusa and Poseidon (also an arch-rival of Athena's since he vied for dominance over Athens, Athena offering the olive tree, Poseidon, the horse), she turned Medusa's beautiful hair into snakey tendrils and banished her to the far ends of the earth beyond the Hyperborean lands where she remained with her sisters.
Meanwhile, the Argosian king, Acrisius, heard an oracle that told him that if his daughter, Danae, gave birth to a son, this grandson would kill him. In an effort to defy the oracle, he locked Danae up in a brazen tower. There Zeus came to her as a shower of gold, impregnating her. Incarcerated, Danae gave birth to a son, Perseus. When the king Acrius heard the clamor of the child, he realized that his efforts to imprison his daughter were in vain. He locked Danae and her new born son Perseus up in a wooden chest, and cast them into the sea.
Drifting in the sea, the fisherman Dictys scooped them up with his net and took the mother and child to the island of Seriphos, where his brother Polydectes reigned. There Perseus grew. Some say that Polydyctes wanted to marry Danae, and it was to save his mother from the marriage that Perseus offered to go off and slay Medusa, while others say that since all guests brought a horse as a marriage gift, Perseus, having no gift, offered to go off and bring Medusa as a fitting wedding gift for Polydyctes. Whatever the reason, Perseus left the island of Seriphos and set off, intent to return with Medusa.