The king of Argos locked his only daughter Danae in a tower where she would never have a baby.
While Danae was locked in her tower. Zeus thought Danae was stunning. So he made it rain and as the rain fell on Danae the magical powers began growing a baby inside her. 9 Months later she gave birth to her son Perseus.
When the King learned of his grandsons birth he was mad and locked his daughter and her son in a chest and threw them in the ocean. The chest stopped at a desert island and a fisherman found it and brought the chest to his brother King Polydectes and they took them in their palace.
Polydectes gave Perseus many tasks. One of them was to kill Medusa. With the sword made from Hermes and the shield given to him by Athena he killed Medusa.
Persues heard the tales of his grandfather and wanted to go see him. So Persues set sail and went to see his grandfather. But before Persues could arrive his grandfather heard that he was coming so he ran away because he was still scared of the prophecy.
While waiting for his grandfather to come back Perseus attended a festival and since he was a skilled athlete he decided he wanted to play in the disc contest. When he was about to throw it he lost control of the disc and the disc went flying into the crowd killing a spectator who turned out to be his grandfather. Which means the prophecy was right.