The King of Greece was told by an oracle that he would be killed by his own grandson, so he sent his grandson and granddaughter away as he thought he could cheat the Gods.
The two children landed of an island and they were cared for by the ruler of the island, and as the years went by the King's grandson (Perseus) grew up to be a strong and active young man.
The king got afraid that Perseus would avenge himself so he sent him on a dangerous quest to bring back the head of Medusa
Perseus set of however soon came across Athena and Hermes who gave them a helmet that would make him invisible, shoes with wings and a shiny shield so Perseus wouldn't have to look at Medusa otherwise he would turn to stone.
Perseus entered the cave of nymphs and came across a sleeping Medusa. He turned around and used his shield so he is able to see Medusa but not change to stone.
Perseus was successful in his quest and returned home with Medusa's head