Acrisius, the king of Argos, is worried about not having any sons to take the throne, so he goes to the Oracle, who tells him that he will be killed by his grandson. In panic, Acrisius locks away his only daughter, Danae, so that she can never have children. Danae prays to Zeus, who hears her and falls in love. He decides to visit her as a shower of gold so that he can get into her cell. In a few months, the baby Perseus is born.
When Acrisius finds out that Danae had a child, he locks her and Perseus in a wooden box and casts them out to sea, so that the Gods cannot punish him for killing family. Danae prays to Zeus again and he convinces the gods to give her and Perseus safe passage across the sea to the nearest kingdom. When the box washes up, a fisherman named Dictys finds Danae and Perseus and takes them to the king, Polydectes, who allows them to stay.
Polydectes falls in love with Danae, but can never get close to her, as Perseus always interferes. When Perseus is finally old enough, Polydectes sends him on an impossible quest - if Perseus can't bring him the head of Medusa, Polydectes will marry Danae.
Perseus is aided by Athena, who gifts him a polished shield and tells him to find the Graiae sisters. Perseus finds them and they give him the winged sandals of Hermes, the Helm of Hades, a bag to conceal the head and a sickle (curved sword). There is another version of the story where Hermes gives Perseus the sword.
Perseus finds Medusa and uses the reflection of his shield to safely sneak up on her and cut off her head while she is asleep.
Perseus returns with the head of Medusa (which still works) and turns Polydectes and his supporters to stone, meaning that Danae is free.