You must go and kill Medusa but first you must find the location of the Gorgons. You must first find the Graeae
Acrisius was told by an oracle that his will be because of his daughters son. So he locks up Danae hoping that he doesn't die from his daughters son. Then Zeus entered her prison as a ray of light. That was how Perseus came to life.
Acrisius finds out that Danae has a baby and sends her and Perseus out to sea and if she dies he will blame it on Poseidon for not keeping the seas safe. They arrive at the island of Seriphos met the king,Polydectes.
Perseus was told to bring a gift to Polydectes dinner party. He disobeyed Polydectes and did not bring a gift. But instead insisted to bring Medusa's head. Everyone in the room started laughing. Then stormed out of the room and goes of to start his quest to find Medusa's head.
Perseus arrives at the Garden of Hesperides to get the Helm of darkness. Then he goes to the Gorgons location and kills Medusa and wraps the head up in goat skin.
Perseus is traveling back home and comes across Andromeda. She was locked up on rocks because she was the cause of tsunamis and the serpent attacking the island. After killing the serpent he carries Andromeda home.
When Perseus arrives he goes into Polydectes palace and shows him and his friends the head of Medusa. Then Perseus,Andromeda and Danae sailed back to Acrisius's palace. He was not there but in a small town called Larissa. A sports festival was on when he came and asked to join in. In discus he threw the discus at Acrisius and killed him and screamed "who can avoid the decree of the gods?". The moral of this story is to not try to avoid your fate as it will happen in the end.