Acrisius had a daughter named Danae who he locked up because of an oracle that his grandchild would kill him but she had a child by Zeus.
So Acrisius sent her and his grandchild, Perseus to an island. Polydectes who was the king of the island wanted to marry Danae but first had to get rid of Perseus so he sent him to kill Medusa.
Perseus goes to the Graeae to find out where medusa is at; he gets it by taking their eye. They gave Perseus the information to get the eye back so they could see.
The Nymps gave perseus an invisibility cap, mirage shield, an sword of infinity. Perseus would be able to kill Medusa using these items.
While medusa was asleep Perseus killed her using the sword of invincibility to cut off her head. But, to be able to see her he used o shield of mirage, and to escape the other two Gorgons he used the cap ofinvisibility.
Perseus then returned home and killed King Polydectes by showing him Medusa's head to save his mother from marriage. Perseus also fulfilled the oracle by killing his grandfather Acrisius who was in the audience of a contest.