The king heard a prophecy that his grandson will kill him so he sends his daughter and the baby out to sea in a wooden chest. The chest washed to shore at the hands of a fisherman.
The brother of the fisherman was a cruel king who wanted to marred Acrisius but the only thing in his way was her son Perseus. The cruel king sent him to fetch the head of Medusa.
Perseus met the Graeae and takes their eyeball. He will not give it back until he gets directions to the gorgons.
He eventually makes it to her and chops her head off in her sleep. He puts the head in his wallet and starts returning home.
As he is returning to the king he saves his future wife from a monster by showing it the head and turning it to stone.
He gets home and realizes that the king sent him to get killed so that he could marry his mother. He saves his mother and a few years later he is throwing darts in a competition and hits a guy in the crowd that happens to be his grandfather.