The Ordinary World: This is Perseus. He's stuck on this island because his grandfather was told by an oracle that Perseus would kill his grandfather. His grandfather put Perseus and his mother in a chest and threw the chest into the sea. Perseus and his mother washed up on an island and lived on the island for the next several years.
The Call To Action: The ruler of the island, Polydectes, fell in love with Perseus' mother. Perseus did not want him to marry his mother so he defended her. Polydectes pretended to marry another princess and asked for wedding gifts. Perseus was too poor to afford a gift so Polydectes ordered Perseus to kill Medusa and bring back her head instead in attempt to get rid of Perseus.
The Refusal To Call: Perseus was afraid to go for Medusa because he knew the possibility of him coming home alive was slim. He felt trapped by his promise to bring back her head. Perseus knew that if he made the mistake of looking at Medusa's face that he would turn to stone like many other victims.
Approach To The Inmost Cave: Perseus became confident in his journey to take out Medusa. Hermes' sword, Athena's shield, and the three magic gifts along with guidance from Athena and Hermes gave Perseus the thought that he can slay the head of Medusa. With this confidence he remembered to not look Medusa directly in the eyes, and to use his shield as a mirror.
The Ordeal: Perseus hovers over the sleeping Gorgons and flies within inches of Medusa, risking death. With his sword at the ready, Perseus chops off Medusa's head, and completes his objective. He stuffed Medusa's snake riddled head into his magic wallet, and flew away with his cap of invisibility, while the other Gorgons pursued Perseus.
The Reward: Perseus now had Medusa's head in his possession, which was his goal in the first place. Now he can return home with his treasure, Medusa's head, and give it to Polydectes as his wedding gift. Polydectes could not marry Perseus' mother, since Perseus will retaurn home with the head of the Gorgon.