Dionysus, the god of wine, prophecy, religious ecstasy, and fertility, returns to his birthplace in Thebes in order to clear his mother's name and to punish the insolent city state for refusing to allow people to worship him. The background to his return is presented in the prologue, in which Dionysus tells the story of his mother, Semele, once a princess in the royal Theban house of Cadmus. She had an affair with Zeus, the king of the gods, and became pregnant. As revenge, Zeus's jealous wife Hera tricked Semele into asking Zeus to appear in his divine form. Zeus, too powerful for a mortal to behold, emerged from the sky as a bolt of lightning and burnt Semele to a cinder. He managed, however, to rescue his unborn son Dionysus and stitched the baby into his thigh. Semele's family claimed that she had been struck by lightning for lying about Zeus and that her child, the product of an illicit human affair, had died with her, maligning her name and rejecting the young god Dionysus.
The action of the play begins with Dionysus's return to Thebes years later. He arrives in town disguised as the stranger, accompanied by a band of bacchants, to punish the family for their treatment of his mother and their refusal to offer him sacrifices. During Dionysus's absence, Semele's father, Cadmus, had handed the kingdom over to his proud grandson Pentheus. It was Pentheus's decision to not allow the worship of Dionysus in Thebes. Dionysus tells the audience that when he arrived in Thebes he drove Semele's sisters mad, and they fled to Mt. Cithaeron to worship him and perform his rites on the mountainside.
As the ruler of the state and preserver of social order, Pentheus finds himself threatened by the Dionysian rites bringing the women from the city into the forest. Unconvinced of their divinely-caused insanity, he sees their drunken cavorting as an illicit attempt to escape the mores and legal codes regulating Theban society. His response is therefore a political one, as he orders his soldiers to arrest the Lydian stranger and his maenads, whom he sees as the root of the troubles. Deviously, Dionysus allows himself to be easily arrested and taken to Pentheus with the others. In the first of three encounters, Dionysus begins the long process of trapping Pentheus and leading him to his death. The encounter begins with the powerful Pentheus thinking he has caught the delicate stranger. He orders his androgynous prisoner to be chained, bound, and tortured but soon finds it impossible to do so. When Pentheus tries to tie Dionysus he ties only a bull, when Pentheus plunges a knife into Dionysus the blade passes only through shadow. Suddenly an earthquake shakes the palace, a fire starts, and Pentheus is left weak and puzzled.
In their second exchange, Dionysus tries to persuade Pentheus to abandon his destructive path, but Pentheus does not relent. A cowherd arrives and describes his sighting of the maddened women of Cadmus. All the women were seen resting blissfully in the forest, feasting on milk, honey and wine that sprang from the ground. They played music, suckled wild animals and sang and danced with joy. But when they saw the cowherd, they flew into a murderous rage and chased after him. The cowherd barely escaped, but the herd of cattle was captured and torn apart by hand by the maenads, including Pentheus's mother Agaue.
Pentheus is left intrigued and excited by the messenger's marvelous and frightening tale. Dionysus takes note of Pentheus's interest and offers him a chance to see the maenads for himself, undetected. Pentheus, on the verge of launching a military expedition to arrest the band, suddenly cannot resist the opportunity to see the forbidden. He agrees to do all Dionysus suggests, dressing himself in a wig and long skirts. The effeminate Pentheus, stripped of his masculinity and authority, is revealed as a vain, boastful and lecherous creature. Once in the woods, Pentheus cannot see the bacchants from the ground, and wants to mount a tree for a better vantage. Dionysus miraculously bends a tall fir tree, puts Pentheus on top, and gently straightens the tree. At once the maenads see him, and Dionysus orders them to attack the vulnerable ruler. With rolling eyes and frenzied cries the women attack, bringing Pentheus down and dragging him to the ground. As he falls Pentheus reaches out for his mother's face and pleads with her to recognize her son. But Agaue, driven mad by Dionysus, proceeds to rip her son to death.
At the palace the chorus is exultant and sings the praise of Dionysus. Agaue returns home with Pentheus's head in her hands. She is still deluded and boasts to all about the young lion she hunted and beheaded. Old Cadmus, who knows what has happened, sadly approaches his daughter and draws her mind back to the palace, her family and finally what she is holding in her hands. Agaue begins to weep. Cadmus remarks that the god has punished the family rightly but excessively. In the end, Dionysus finally appears in his true form to the city. He banishes Agaue from Thebes and ordains that Cadmus and his wife will turn into snakes, destined to invade Greek lands with a horde of barbarians.