Semele was born in Thebes, the great city of Boeotia in central Greece, to Cadmus and Harmonia. Cadmus had come from Tyre in Phoenicia, on the eastern coast of the Mediterranean, searching for his sister Europa, who had been carried away by Zeus in the form of a white bull, and had founded Thebes after slaying a dragon that guarded a sacred spring and sowing its teeth into the ground. From those teeth sprang the Spartoi, armed men who fought one another until only five remained, and from those five came the noble families of Thebes. Harmonia was the daughter of Ares and Aphrodite, and their marriage had been blessed by the gods with a necklace of divine craftsmanship, a gift that brought both glory and sorrow to all who wore it. Semele grew up in the palace of this legendary house, beautiful and proud, and when she came of age she caught the eye of Zeus himself.

Zeus came to her in secret, as he often came to mortal women, disguised as a man or wrapped in a golden cloud. Their affair was conducted in the palace or in a nearby meadow, and Semele conceived a child. But Hera, the wife and sister of Zeus, the queen who sat beside him on Olympus and who had persecuted every mortal woman her husband had ever loved, discovered the affair. She could not strike at Zeus directly, for he was king of the gods and wielder of the thunderbolt, but she could destroy the mortal who had dared to share his bed. Hera devised a plan of exquisite cruelty. She disguised herself as Beroe, the aged nurse of Semele, and came to the princess with a face wrinkled by false years and a voice trembling with feigned concern. She told Semele that many men claimed to be gods, that kings and princes boasted of divine parentage to seduce innocent girls, and that Semele should test her lover. If he truly was Zeus, she should demand that he appear to her in the full glory of his divinity, just as he appeared to Hera herself on their wedding night.

Semele, young and uncertain, took the nurse's advice to heart. The next time Zeus came to her, she asked him to swear an oath by the River Styx, the sacred water that flowed around the Underworld and whose oath was binding even upon the gods, that he would grant her any request she made. Zeus, loving and unsuspecting, swore the oath. Semele then asked him to appear to her in his true form, not as a man or a cloud but as the king of the gods in all his power and radiance. Zeus was horrified. He begged her to choose anything else, gold or immortality or kingdoms, but Semele held him to his oath. Bound by the unbreakable promise, Zeus could only comply.

He came to her palace not as a lover but as the storm itself. The sky darkened, thunder rolled across the heavens, and lightning split the air. Zeus appeared in his true form, wreathed in clouds and blazing with the light that no mortal eye could bear. The palace shook, the walls cracked, and Semele, looking upon the face of the god she had loved, was consumed by the fire of his presence. The thunderbolt that was his weapon and his essence struck her, or the sheer radiance of his divinity burned her to ash where she stood. She died in an instant, her body consumed by holy fire, and only her womb remained intact, protected by Zeus's power because it carried his unborn son.

Zeus reached into the flames and seized the infant Dionysus, still no larger than a six-month child, for Semele had been in the sixth month of her pregnancy. To save the child, Zeus cut open his own thigh, placed the infant within, and sewed the wound shut with golden thread. For three more months, Dionysus grew inside the body of his father, fed by Zeus's immortal blood, until the time came for his birth. When the thigh was opened, Dionysus emerged, fully formed and divine, the only Olympian god to have been born twice, once from a mortal mother and once from an immortal father. The child was given to Hermes, the messenger god, who delivered him to the nymphs of Mount Nysa, where he was raised in secret from Hera's wrath.

Semele's story did not end with her death. For years she wandered as a shade in the Underworld, the realm of Hades, the god of the dead, and Persephone, his queen, among the countless ghosts of mortals who had died without honour. But her son Dionysus, when he had grown to manhood and established his worship across the world, descended into the Underworld to retrieve her. In some versions, he went to demand her release from Hades, and the king of the dead could not refuse the god who had conquered both life and madness. In others, Dionysus simply took her by the hand and led her up through the dark passages into the light. He brought her to Olympus, the sacred mountain where the twelve great gods made their home, and there Zeus granted her immortality. She was given the divine name Thyone, and she became a goddess in her own right, honoured alongside her son in the rites of the Dionysiac mysteries. The Thyiades, the Maenads who worshipped Dionysus at Delphi, were named for her, and every winter when they climbed Mount Parnassus in the snow, they carried her memory with them.

Semele was worshipped at Thebes, her native city, where her tomb was shown to visitors and where the sanctuary of Semele stood near the palace of Cadmus. The Thebans honoured her as the mother of their most famous divine son, and her story was told in the tragedies of Euripides and the hymns of the Orphic tradition. She was also identified with the moon in some mystical traditions, and her death by fire was interpreted as the burning away of mortality to reveal the divine spark within. From the ashes of her palace to the golden thigh of Zeus, from the darkness of the Underworld to the heights of Olympus, her journey was the most dramatic transformation any mortal woman had ever undergone.