Perseus and his mother, Danae, live on the island of Seriphus with a fisherman named Diktys. He's the son of Zeus, but that's another story all together.
Diktys' brother, Polydectes, is the king of island, and he totally has a thing for Danae.
Polydectes is worried that Perseus will interfere with him marrying Danae. To get Perseus out of the way, Polydectes hatches a scheme.
He gathers a bunch of his friends (Perseus included) and claims that he's collecting presents to win the hand of a lovely lady named Hippodameia (even though the king really has his eye on Danae). Polydectes asks each man to give him some horses.
Perseus doesn't have a horse to give, but he brags that he will bring Polydectes anything else he wants.
"Oh really?" says Polydectes. "Great, bring me the head of a Gorgon."
At this point, Perseus probably pees in his pants a little bit. The Gorgons are three monstrous sisters with snakes for hair, bronze hands, wings, and tusks. They're so hideous that anyone who looks at them turns to stone. It seems like Polydectes is sending Perseus on a suicide mission.
Even though the quest he's been assigned seems impossible, Perseus sets out to get the job done.
With the help of the gods Athena and Hermes, Perseus locates the Graeae, three nasty old hag sisters who share one eye and one tooth between them. (Gross. There are some things you just shouldn't share with your siblings – teeth and eyeballs are near the top of that list.)
Perseus steals the eye and the tooth from Graeae (ew).
Our noble hero won't let the old ladies have their precious eye and tooth back until they tell him the location of the Nymphai, three nymphs who have some of the gods' treasures in their safekeeping.
The hags do as Perseus asks, and he gives them back their body parts.
Following the Graeae's directions, Perseus finds the Nymphai, who let him borrow several awesome items:
the wing sandals of Hermes, which allow him to fly;
Hades' helmet of invisibility (a.k.a. Helm of Darkness), which makes him invisible;
and the kibisis, a knapsack tough enough to hold the toxic head of a Gorgon.
Perseus puts on all his new toys and flies off to face the Gorgons.
Along the way he runs into the goddess Athena, who advises him to only look at the Gorgons through the reflection of his shiny bronze shield, otherwise he'll be turned to stone.
Eventually, Perseus reaches the seaside cave of the three Gorgon sisters, whose names are Stheno, Euryale, and… Medusa.
Luckily for Perseus, the Gorgons are asleep when he gets there.
Perseus sneaks up to Medusa, the only Gorgon who is mortal. He raises his sword, taking aim by looking at the monster through the reflection in his shield, and chops off Medusa's head.
As Medusa's head flies from her body, Pegasus, the winged horse, and Chysaor, a giant with a golden sword, jump out of her neck. (This is not quite as random as it sounds. There's a good explanation, but that's another story.)
Perseus plops Medusa's head in the kibisis and starts make his getaway.
At this point Medusa's sisters, Stheno and Euryale, wake up, and they are not happy. They chase after Perseus.
The hero puts on Hades' helmet of invisibility and escapes the crazy snake ladies.
Perseus then flies away with Medusa's head packed in the kibisis.
On the way back to the island of Seriphus, Perseus runs into Atlas, the Titan doomed by Zeus to hold up the sky forever.
The hero and the Titan get into a fight, because Atlas refuses to offer Perseus shelter.
Hotheaded Perseus ends the argument by whipping out Medusa's head and turning Atlas into stone.
(It is said that Mt. Atlas, in modern day Morocco, is the petrified remains of the Titan.)
Perseus gets into more shenanigans, on the way back. When he flies over Egypt, some of the Gorgon's toxic blood drips into the desert sand, creating a breed of really poisonous snakes. (Wow, thanks Perseus.)
When Perseus is flying over Africa, he notices the beautiful maiden, Princess Andromeda, tied to a rock by the sea. He ends up rescuing her from a sea monster and making her his wife. (For much more on this famous story, click here.)
When Perseus finally makes it back home to Seriphus, he finds out that his mother is hiding out in a temple trying to avoid the lusty king Polydectes. Perseus marches over to Polydectes' palace, where he finds the king hanging out with some friends.
Polydectes is all, "Hey, what are you doing here? Where's Medusa's head?"
Perseus says, "Here it is, jerk," and he turns his face away and reveals Medusa's severed head.
Polydectes and his buddies are all turned to stone. Perseus' mom is now safe.
After this, Perseus returns all his godly artifacts – winged sandals, kibisis, helmet of invisibility – to the Nymphai.
As a token of thanks, Perseus gives Medusa's head to Athena, who places it on the front of her shield.