A king was furious because he couldn't have a child. He asked the Oracle if he would have a son. The Oracle replied that he would have a grandson and a daughter, and his grandson would kill him.
In great fear of being murdered, the king locked up his daughter in a chamber in the sea. One day, golden rain fell into the chamber. That rain was Zeus. The king's daughter gave birth to a son, Perseus.
The king was now aware of his grandson, so he threw his daughter and Perseus into the ocean. They washed up on land, and a new king fell in love with the other king's daughter. The king didn't want Perseus to interfere with his wedding, so he set Perseus off on a journey for a Gorgon head.
So, Perseus set off on an adventure to go look for a Gorgon head. Hermes gave him an unbreakable sword, and Athena gave him a shield to see the Gorgon Medusa's reflection in, since looking into Medusa's eyes petrified you. (Turned to stone)
Perseus hadn't had a clue where Medusa was, until he visited the Grey Sisters. They shared an eye to see. Perseus outsmarted them by taking the eye and threatening to destroy it. Frantic, they told him where Medusa slept. He travelled to her cave.
Perseus had done it. He'd slain Medusa, and returned with a Gorgon head. Medusa's two sisters were overcome with rage, immediately wanting to avenge their lost sister. But Perseus came home victorious.