Some background is helpful in understanding the events of the play. Semele, princess of Thebes and daughter of Cadmus, was impregnated by Zeus, king of the gods. She boasted proudly of the child's fathering, and her sisters became jealous. More importantly, the girl's pride offended Hera, Zeus's wife and queen of the gods. Because of Hera's anger, Semele was struck by lightning as she gave birth. Zeus preserved the child and hid him from Hera. The child was Dionysus, and he grew into a powerful deity. In Thebes, Semele's own sisters slandered her, saying that the girl was no bride of Zeus; clearly, said Semele's sisters, the lighting was punishment for Semele's lies. The child was assumed to have perished with its mother. But in truth Dionysus was growing into a powerful god, traveling around Asia and teaching his rites to man.
As the play begins, Dionysus has returned to Thebes to punish Semele's sisters. He has brought with him a host of Asian followers, and he means to initiate Thebes into his new religion. He has begun by possessing all of the women of Thebes. They are now the Maenads, possessed by the ecstasy of Dionysus. The women have left their homes and now celebrate wildly in the wilderness of Cithaeron. Among the women are Semele's sisters, including Agave, mother of Thebes' current king. Cadmus is still alive, but he is very old and has abdicated in favor of his grandson Pentheus. Pentheus, though probably still a teenager (his exact age is never given), rules as king. He has denounced the new religion. Before exiting, Dionysus announces his intentions to punish the disbelievers.
Teiresias and Cadmus enter, preparing to follow the Asian Bacchae (followers of Dionysus) into Cithaeron. They are the only two men who have decided to participate in the rites of the new god; because of the other men, they fear that punishment will fall on Thebes. Teiresias and Cadmus are both somewhat pathetic and vulnerable figures, old men surrounded by young, strong women. Pentheus enters, denouncing the new religion and the disorder it has brought to Thebes. He becomes infuriated when he sees Teiresias and Cadmus dressed like worshippers of Dionysus. Both men argue with the young king, defending the pragmatic wisdom of embracing the new religion. But Pentheus refuses to listen to them, and he becomes more determined than ever to put an end to the new cult. He has heard that there is a priest who is at the center of the new religion, and Pentheus intends to have the man captured and possibly executed: the audience knows that this priest is Dionysus himself, disguised as a mortal.
Attendants bring Dionysus, captive, to Pentheus. Pentheus interrogates Dionysus, who Pentheus thinks is a mortal man; Dionysus' answers are slippery and meant to madden the king. He shaves away the god's curls and has him put in the dungeon. The Bacchae will be enslaved.
Lightning strikes and there is a mighty earthquake: the royal palace of Thebes is completely destroyed. Dionysus emerges from the rubble, still disguised as a mortal; he speaks to the fearful Bacchae, telling them that the god has brought this destruction as punishment. Pentheus comes out from the rubble, outraged. He means to punish the priest horribly. A messenger enters with terrifying news for Pentheus: the Maenads in the hills have powers beyond those of mortals. They have destroyed villages in the foothills, routing soldiers almost without effort. Pentheus means to take his best troops into the hills to make all-out war against the Maenads.
But Dionysus calls out to him, asking him if he would like to see the women's revelry. The boy is fascinated by the idea, and he exits to consider Dionyus' proposal. Dionysus insists that Pentheus must go disguised as a woman. When Pentheus returns, he is dressed as Dionysus instructed; he also is dazed, now completely possessed by the god. They leave to observe the Maenads.
Before long, a Second Messenger brings news of what has happened: Pentheus is dead. Dionysus revealed him to the Maenads, who ripped the boy to pieces with their bare hands. The first to attack the boy was his own mother, Agave. She is possessed, and thinks that she killed a lion. She now enters, with Pentheus' head impaled on a thyrsus. She boasts to the Bacchae about her prize.
Cadmus enters, with attendants carrying a bier on which lie the shredded remains of Pentheus' mutilated body. The old man rushed to the hills as soon as he heard about the murder. He has also heard that his daughter is still possessed by the madness of the god.
Agave boasts to her father that she killed a lion with her bare hands. Cadmus speaks with her gently, trying to help her return from the madness of the god. When the ecstasy has passed, Cadmus tells her to look at what she carries in her hands. Agave shrieks: she cannot remember killing Pentheus. Cadmus tells her what happened. Father and daughter grieve together, and even the Bacchae are moved to pity.
Dionysus appears above the palace, now in full glory as a god. He proclaims that Agave and Cadmus are to be exiled, separately. Cadmus begs for mercy, but to no avail. Agave and Cadmus grieve for Pentheus, for themselves, and for each other. They exit separately, destined to face still more hardship.