Daughter of Phorcys and Ceto, and therefore sister of the Graeae, but mortal, Medusa is a beautiful young girl with whom Poseidon falls in love. Raped by this god in a temple dedicated to Athena, she is punished by this same goddess who transforms her into a Gorgon. Her hair becomes snakes, her eyes dilate and from then on her gaze petrifies all those who cross it. According to another version, Medusa was a young girl so proud of her beauty and her hair that she dared to rival Athena. To punish her, the goddess changed her hair into snakes and modified her gaze. Similarly, according to Ovid "among all her charms, what most charmed the eyes was her hair".

Danaë, driven out of Argos with her son Perseus, is taken in by a fisherman from this island. He takes her into his house and raises her child. But Polydectes, the fisherman's twin brother and king of the island, had fallen in love with Danaë and was looking for a way to get rid of the young Perseus, he asks him for "the head of Medusa!", if he does not bring it to him, he could then marry his mother Danaë. Lamenting his sad fate, Perseus goes to the other end of the island. He meets the god Hermes, who tells him not to be discouraged. With his help and that of Athena, Perseus goes to the sisters of the Gorgons, the Graeae (the "Old Ones"), who are born with white hair and live in a cave where the light of the Sun or the brightness of the Moon never penetrates; They have only one eye and one tooth together, which they pass around in turn. When they refuse to cooperate, he steals their eye and tooth at the moment to force them to answer him. He thus learns the path that leads to the nymphs. When he arrives there, they show him the cave where the Gorgons are and give him three magical objects: winged sandals that allow you to fly, the helmet of Hades that makes you invisible because it is the "mask of a deceased person", as well as a bag to place Medusa's head in.

Thus equipped, Perseus flies to the shore of the Ocean and arrives in the cave where the three Gorgons are sleeping. They are monstrous, with their heads covered with scales, their boar's tusks, their tongues hanging out, their bulging eyes, their wings and hands of bronze, as well as snakes around their chests. Whoever looks at them is turned to stone. Covered with the helmet of invisibility and taking care to turn his head away, the hero succeeds in approaching them thanks to the reflection that his shield polished like a mirror sends back to him. He seizes Medusa gropingly and decapitates her. From Medusa's severed neck spring two sons, Chrysaor, father of Geryon, and Pegasus, the winged horse.

Perseus puts Medusa's head in his bag and flees using his winged sandals, while being hidden, thanks to his helmet of invisibility, from the two other Gorgons who pursue him while crying. Continuing his journey, Perseus sees the virgin Andromeda tied naked to a rocky islet as a sacrifice to Ketos, a sea monster sent by Poseidon. He frees her, falls instantly in love with her and ends up marrying her, after having obtained the parents' agreement by killing the monster that was ravaging their country. In passing, the algae around the rock have been petrified into coral. As Andromeda's uncle, Phineus, picks a fight with her by claiming that the young girl was promised to him. Perseus petrifies him using Medusa's head.

Returning with Andromeda, he learns that Danaë and the sinner have taken refuge in a temple to escape the advances of Polydectes accompanied by his army. He petrifies them, establishes the sinner Dyctis as king and offers Athena the head of Medusa, which the goddess, with the help of Hephaestus, fixes on her head.