When Hera finds out that Zeus is cheating on her (i.e. doing what he does best) with a Theban princess named Semele, the queen of the gods is out for revenge. In disguise, Hera tells the girl, who's pregnant with Zeus's son, Dionysus, to ask Zeus to appear to her in the same form that he does to his wife.
Zeus unwillingly complies to the request and appears as a great ball of fire, incinerating his lover. Just in time, though, he scoops the unborn Dionysus from his mother. The king of the gods then sews the boy into his thigh (ouch?) until he's ready to be born. After this, Dionysus is raised by some friendly nymphs on Mt. Nysa and the satyr, Silenus, until he's ready to go spread his wild, wine-swilling cult around the world.
The Less Short Story
Zeus is up to his old tricks again: having an affair with yet another beautiful girl.
This time, the (un)lucky girl is Semele, a princess of Thebes.
The affair has been going on a little while, and Semele is even pregnant with Zeus's baby, a godling who will one day be called Dionysus.
The king of the gods is really into Semele and tells her that he'll grant her one request, whatever it is.
Semele tells her godly lover that she'll have to think about that one.
When Zeus's wife, Hera, hears about her husband's affair, she says to herself, "Girl, you know you've all about revenge. But this time, I'm gonna go extra hard in the paint."
So, Hera goes in disguise to Semele (either as an old lady or as a friend of the princess).
The queen of the gods, using the utmost of her cunning, plants a nasty suggestion in Semele's mind: she tells Semele that she ought to ask Zeus to come to her in the same form that he appears to his wife.
This (unfortunately) sounds like a great idea to Semele.
Hera sneaks away, thinking she really outdid herself this time.
So, when Zeus shows back up, Semele says that her one request is to see him in the true godly form that Hera sees.
Zeus is all like, "Baby, I really don't think you want that."
"Ah, come on Zeusy-poo," says Semele, stamping her left foot. "You promised me anything."
"Don't say I didn't warn you," Zeus tells her with a shrug, and then transforms into a great ball of fire.
Semele is shocked and awed and then is incinerated.
(Seems like Zeus could've tried a little harder to talk her out of it, right?)
Just in time, Zeus plucks Dionysus's fetus from Semele's ashes.
(This just got a little graphic.)
Dionysus's birth is premature, so Zeus sews him into his thigh until the godling is fully developed.
When the time is right, baby Dionysus hops out of from his dad's leg like a pie out of an Easy-Bake Oven, ready to rock the world.
Zeus is a hands-off kind of dad (understatement of the year), so he gives the little bugger to Hermes and says, "Take care of this."
Hermes takes the godling to Mt. Nysa, where some friendly nymphs, the Nysiades, raise him.
(It's said that Zeus later honored the Nysiades by turning them into the constellation, the Hyades.)
While Dionysus is growing up in the mountain wilderness, he becomes best buds with Silenus, the satyr, and his gang of satyr buddies.
It isn't long before Dionysus discovers the grape vine and the process of making wine.
Soon his nymph and satyr pals are partying hard with him in the wilderness, and the seeds of all the wild Bacchanals that will become his trademark are planted.
Some say that young Dionysus travels to the Underworld at some point, retrieving Semele and transforming her into the goddess, Thyone.
(Pretty impressive, for a Mama's boy.)
Other stories say that Dionysus is driven insane by Hera for a little while and travels far and wide in his madness.
Most agree that Dionysus goes all over the world spreading his cult— mad or not.
Next, he travels throughout the Mediterranean and the Middle East inspiring people to worship him and punishing (terribly) those who don't.
The young god even makes it as far as India, where he wages holy war on the people there until they see the light and join in his Bacchanals.
Eventually, Dionysus decides it's time to spread his cult to the land of his mother.
So, he rounds up his pack of wild followers and heads to Thebes.
Of course, he's not really all that pleased with what he finds there....