Pentheus was born to Agave, the daughter of Cadmus, the founder of Thebes, and Harmonia, the daughter of Ares, the war god, and Aphrodite, the goddess of love. His father was Echion, one of the Spartoi, the armed men who had sprung from the dragon's teeth sown by Cadmus in the plain of Thebes. The Spartoi had been born fully armed and had immediately turned upon one another, fighting until only five survived, and Echion was one of those five, a man of violence and war who married into the royal house. Pentheus inherited both the throne and the pride of his lineage. When Cadmus grew old and retired, Pentheus became king of Thebes, a city whose walls were legendary and whose royal family was marked by divine favour and divine curse in equal measure. He was a young man, strong and arrogant, convinced that his authority as king was absolute and that no god could challenge the order he imposed upon his city.

The crisis came when Dionysus returned to Thebes. Dionysus was the son of Zeus and Semele, a Theban princess who had been consumed by divine fire when she demanded to see Zeus in his true form, and he had been born from Zeus's thigh after her death. He had grown to manhood in the wild mountains of Nysa and had travelled the world spreading the cultivation of the vine and the ecstatic worship that accompanied it. When he came to Thebes, he found that his mother's sisters, the daughters of Cadmus, had spread the rumour that Semele had lied about her union with Zeus and that Dionysus was not a god at all but the illegitimate child of some mortal man. Pentheus, who was the cousin of Dionysus through their shared Theban ancestry, embraced this denial. He banned the worship of Dionysus in his city, declared that the god was a fraud, and imprisoned the women who had fallen under the god's spell, the Maenads, the frenzied female followers who had abandoned their homes to dance on the mountainside in a state of divine possession.

Dionysus came to Thebes in disguise, a beautiful young stranger with long curling hair and wine-dark eyes, and Pentheus had him arrested and brought before the throne. The god allowed himself to be bound and imprisoned, but his chains fell away of their own accord, and the prison doors opened for his followers. Pentheus, now consumed by a mixture of contempt and morbid curiosity, demanded to see the rites of the Maenads for himself. Dionysus, playing upon the king's arrogance, offered to show him the secret worship on Mount Cithaeron, the mountain that rose above Thebes, promising that Pentheus could spy upon the women from the trees without being seen. Pentheus agreed, and Dionysus dressed him in a long woman's robe, arranged his hair beneath a wig, and placed a thyrsus in his hand, the pine-cone staff of the god's worshippers. The king of Thebes, who had forbidden the rites, was now disguised as one of their participants, led by the god he had denied toward the very frenzy he had sought to suppress.

On Mount Cithaeron, the Theban women had been driven into a state of ecstatic madness by Dionysus. They had left their looms and their children, cast off their shoes and veils, and run into the forest with their hair unbound and their voices raised in the high-pitched cry of the ololuge. They danced in circles, whirling faster and faster, and they hunted animals with their bare hands, tearing them apart and eating the raw flesh in a ritual called sparagmos. Among them was Agave, Pentheus's own mother, and her sisters Ino and Autonoe, all daughters of Cadmus, all once respectable women of the royal house, now transformed into the most terrifying instruments of the god's vengeance. Dionysus led Pentheus to a tall pine tree and helped him climb into the branches, and from that height the king looked down upon the Maenads, watching their wild dance with a mixture of horror and fascination.

But the god called out to the women, and they sensed the intruder in the tree. They surrounded the pine and pulled at its trunk, rocking it back and forth until Pentheus fell among them. In his terror he called out to his mother, begging her to recognise him, but Agave, her eyes fixed and her mind lost in the god's frenzy, saw not her son but a wild beast, a mountain lion or a bull. She was the first to seize him, and with the strength of divine possession she tore his arm from his body. The other Maenads fell upon him, ripping his flesh with their hands and teeth, tearing his body into pieces while he screamed. Ino and Autonoe joined the slaughter, and Pentheus, the king of Thebes, was dismembered on the mountainside where he had come to spy. Agave, still in her frenzy, lifted her son's severed head upon her thyrsus and carried it down the mountain in triumph, her face flushed with the joy of the hunt, believing she had killed a great beast and brought honour to Thebes.

She returned to the city and made her way to the palace, where her father Cadmus and the prophet Tiresias, the blind seer who had warned Pentheus to honour the god, were waiting. Cadmus asked where her son was, and Agave held up the head upon the thyrsus and declared that she had hunted the beast herself, a fine trophy for the city. Only when Cadmus gently removed the ivy wreath from her hair did the divine madness lift, and Agave looked upon the face of her son and understood what she had done. The horror of the moment, the mother who had murdered her child at the command of a god she had helped to deny, became one of the most powerful and terrible scenes in all Greek tragedy. The palace fell into mourning, and the royal house of Thebes, already marked by the curse of the dragon's teeth and the violence of the Spartoi, was shattered once more by divine vengeance. Dionysus appeared in his true form and declared that the city had learned its lesson, and that his worship would now be established in Thebes forever.