Dionysus is a total party animal. He's the god of wine and revelry so you really can't blame him. Inspiring people to drink vats of wine and dance till they reach a state of total ecstasy is just in his job description. When he pops up in Thebes to get the party started, though, the stuffy King Pentheus gives him some not-so-stiff opposition.
The D-man has a lot to prove when he shows up in Thebes. For one, this is the town his mom, Semele, was from. He knows everybody in Thebes thinks his mom got pregnant from some random mortal dude instead of Zeus. So, when he proves that he's a god by putting all the women in town under his spell, he's also proving that his mom isn't... um... of loose moral character. (Except where Zeus in concerned, which we guess is okay because he's a god and all.)
Getting the Thebans to worship him is also a big deal for Dionysus, because it's the first city in Greece where he's trying to spark up his wine-swilling cult. It might not be quite as easy a sell as you might think. Fun as they sound, the Bacchanals also have a dark side. The rituals were designed to open up the wildest, most primal places in human beings. When that door gets opened, pretty crazy things can happen. You know like breastfeeding wild animals, ripping live bulls apart, eating raw flesh, and (as in Agave's case) tearing the head from your own son. (Yup, that's crazy.)
Poor King Pentheus... he's pretty easy to dislike. We bet nobody played with him on the playground. Dude's a total party pooper, right? This guy is bound and determined to not recognize the wine-lovin' Dionysus as a god. If you look a little closer, though, you might find that the King isn't as big of a stick in the mud as you thought.
Put yourself in Pentheus' position. You've got a nice, orderly city where everything is running smoothly. Then your crazy cousin shows up from wreaking havoc in Asia, saying that he's a god and that everybody in town has to get wasted and join his weird cult. We're guessing most people wouldn't be like, "Oh, sure that sounds like a great idea." So, maybe, just maybe, Pentheus up-tightedness isn't as bad as you thought.
Agave is Pentheus' mom and Dionysus' aunt. Unfortunately, when Dionysus shows up in Thebes, it is not a happy reunion. Agave and her sisters Ino and Autonoe have spent the years since Dionysus' birth spreading dirty rumors about his mom Semele, who was also their sister. Rather than admitting that Semele was impregnated by Zeus, Agave went around telling everybody that it was some random mortal guy. Why did Agave and her sisters do it? We don't know. Maybe she was jealous that Zeus didn't pick her. (Not that that would've been all that cool, since Semele was obliterated for the affair through a trick of Hera.)
To punish Agave for her gossipy ways, Dionysus makes her go crazy, turn into a Maenad, and lead the rest of the ladies in town in the wine-soaked rituals up on Mt. Cithaeron. The final punishment comes when Dionysus makes Agave think that Pentheus is a lion. Fueled by wine and Bacchic fury, Agave rips the head from her own son's body. Dionysus only allows his aunt to come back to reality when she's back in Thebes with Pentheus' head on the end of stick. Needless to say, this is a pretty awful moment for Agave. We have a feeling she won't be spreading nasty rumors again.