The King had talked to the oracle and figured out that one of his grandsons is going to kill him and take over.
To make sure that he would still be King, he put her daughter in a ditch. Then this golden rain came and brought her Perseus. The same thing happened when they got married and figured out that she found a child.
When they got married Perseus forgot a present and told them he would get them the head of Medusa. He started at the land of the oak trees where he met Hermes. He told him that he needed three items which belonged to the nymphs of the nymphs of the north. While he was there he got 2 items: a sword and a shield.
When Perseus arrived at the nymphs he got the three special items: a pair of winged sandals, a bag that could fit anything, and a invisible helmet that made the person who was wearing it dissapear.
When he got there the were all sleeping and Hermes pointed out which one is Medusa. He chopped her head off.
Her sisters were furious but he didn't care because he was invisible then flew home happily.