Dionysus has returned to Thebes after establishing his cult in the East. He comes (disguised as a stranger) to refute Agave's slander that he is no son of Zeus. Every woman of the city he has driven in a mad frenzy to the hills. Chorus of Asian followers sings an ode to Dionysus.

Teiresias and Cadmus appear, in bacchante dress; the old men will dance, for Bacchus makes them young; only they believe in tradition; the clever young are blind. Pentheus (grandson of Cadmus and the king) is outraged at these bacchic goings-on. Teiresias defends myths and mysteries: he interprets the story of Dionysus sewed into Zeus' thigh (a mistake: he showed  a dummy Dionysus to jealous Hera); he admits that Bacchantes indulge in sex, but only to their own inclinations. Pentheus rejects them, and orders the stranger arrested.

Pentheus mocks the fair stranger who has introduced the rites to Thebes. He asks him about the rites. He has him bound and imprisoned.

Thunder. The palace of Pentheus collapses. Dionysus comes forth. Pentheus enters, enraged and baffled by D's escape. A messenger reports describes the wondrous ways of the bacchantes (water from rocks, milk from the ground, honey from reeds), who tear apart calves and bulls, and rout spear-armed men with their wands. He advises submission to the great god. Pentheus will not be humiliated: he orders the army to be summoned, in heavy armor.

Dionysus offers to lead the women back without bloodshed, and entices Pentheus to come along to see their revels. Pentheus, though he hesitates, cannot resist, and agrees to put on woman's dress and a wig of curls. He is now under the god's power. They run to the mountain.

A messenger reports the horrible death of Pentheus, torn apart by the hands of his mother Agave. Agave enters with the head of Pentheus impaled on her thyrsus. She thinks it the head of a lion's cub and glories in her kill. Cadmus sadly enters with the other remains of Pentheus, gathered from the mountainside. Agave slowly comes out of her frenzy and realizes what she has done. They have suffered because they blasphemed the god. Cadmus declares that if any depises or defies the gods, he should look on Pentheus' death and believe.

Dionysus appears (as the god), ex machina. He declares Pentheus' death and their sorrow a just punishment. Furthermore, Agave, now unclean, must be exiled. Cadmus and his wife will be turned into snakes and lead a barbarian host against the Greeks. Cadmus admits his guilt (!), but complains that the sentence is too harsh: gods should be above such passions. Dionysus declares it that all is fixed; this is the fate ordained by Zeus. Chorus: the god has found his way for what no man expected.