When the Oracle told King Acrisios that his grandson was to grow up and kill him, he decided to lock Danae, his only daughter, in a room. Unfortunately, Perseus was born (Zeus' son) so he shipped them away at sea, hoping that they would either drown or starve.
Luckily, King Polydectes's brother, Dictys, found them while he was fishing. King Polydectes and Dictys both fell in love with Danae. Danae preferred Dictys, but because Polydectes was the king, she had no control over who to marry.
The king hated that Perseus was guarding Danae all the time. To get rid of Perseus, he got mad that Perseus had never given him anything, even though the king had raised him as a baby. Perseus agreed to fetch anything for Polydectes, and Polydectes told him to fetch Medusa's head.
Perseus had a vision. Athena was talking to him.
First you must go to the sister Phorcides who will tell you the way to the nymphs, who guard the hat of darkness, the winged sandals, and the knapsack which can hold the Gorgon's head. Then I will give you a shield, and my brother Hermes will give you a sword, which shall be made of adamant, the hardest rock. For nothing else can kill the Gorgon, since so venomous is her blood that a mortal sword, when plunged in it, is eaten away. But when you come to the Gorgons, invisible in your hat of darkness, turn your eyes away from them, and look only on their reflection on your gleaming shield. Thus you may kill the monster without yourself being turned to stone. Pass her sisters by, for they are immortal, but smite off the head of Medusa with the hair of writhing snakes. Then put it in your knapsack and return, and I will be with you.
Perseus followed Athena's directions, and killed Medusa.
Perseus was known as a hero. One day, as he was playing discus with some other heroes, the wind blew the discus and it flew in the crowd. King Acrisios got hit and died of shock. Acrisios' fate was sealed at last.