Leda was born in Aetolia, the mountainous region of western Greece, the daughter of Thestius, the king of Pleuron, and his wife Leucippe. Her father was a descendant of Agenor, the Phoenician king whose daughter Europa was carried away by Zeus in the form of a white bull, and her family was connected by blood and marriage to the greatest houses in Greece. When she came of age, she married Tyndareus, a prince of Sparta who had been driven into exile by his half-brother Hippocoon. Tyndareus fled to Aetolia and found refuge at the court of Thestius, where he met Leda and won her hand. With the help of Heracles, the hero forced to complete twelve impossible labours, Tyndareus was restored to the Spartan throne, and Leda became the queen of the most powerful city in the Peloponnese, the mistress of a palace beside the river Eurotas and the sacred guardian of the Spartan royal line.

The most famous night of Leda's life began like any other. She was bathing in the river Eurotas, or walking beside it, or resting in her chamber, depending on which poet tells the tale, when a great swan descended from the sky. The swan was Zeus, the king of the gods, who had transformed himself into the bird to escape the notice of his wife Hera, the queen of Olympus whose jealousy was legendary. In some versions of the story, Zeus was fleeing from an eagle, the sacred bird of Hera, and Leda took pity on the swan and sheltered it in her arms. In other versions, the swan forced itself upon her, and the encounter was violent rather than tender. In still other versions, the swan seduced her with its beauty, and Leda welcomed the god in disguise. The ancient Greeks told the story in many ways, and the ambiguity of that night - whether it was rape or seduction, divine assault or willing union - has never been resolved.

The result of the union was extraordinary. Leda conceived twice on the same night, once by Zeus and once by her husband Tyndareus, or in some accounts she laid two eggs, each containing twins. From the union with Zeus came Helen, the most beautiful woman who ever lived, whose abduction by Paris, the prince of Troy, would cause the Trojan War and the destruction of the age of heroes. From the same divine union came Polydeuces, also called Pollux, the boxer who was immortal because his father was a god. From the union with Tyndareus came Clytemnestra, the queen who would marry Agamemnon, the high king of the Greeks, and murder him in his bath with the help of her lover Aegisthus when he returned from Troy. And from the mortal union came Castor, the horseman who was mortal because his father was a man. The four children grew up together in the palace at Sparta, and though they had different fathers, they were raised as one family, the royal house that would shape the destiny of Greece.

The relationship between the twins Castor and Polydeuces was the most famous brotherhood in Greek mythology. Castor was mortal, Polydeuces immortal, but they refused to be separated. They sailed with the Argonauts, the crew of the ship Argo who sought the Golden Fleece, and they rescued Helen when she was stolen by Theseus, the king of Athens, before she was old enough to marry. When Castor was killed in a feud with the sons of Aphareus, his cousins Idas and Lynceus, over a cattle raid, Polydeuces begged Zeus to let him share his immortality with his brother. Zeus granted the prayer and placed them both in the sky as the constellation Gemini, the heavenly twins, where they remain to this day as the brightest stars of the winter sky and the protectors of sailors in stormy seas.

Leda's own death is not described in the early sources. Some later poets said that she was deified after her death and placed in the sky near her sons, but the Homeric tradition is silent on her end. What is certain is that she outlived the Trojan War in some accounts, and in others she died before it began, her heart broken by the knowledge of what her daughters would do. The Spartans worshipped her as a goddess, or as a heroine, and her tomb was shown to travellers in the city that bore her name. The Roman poet Ovid described her in his Metamorphoses with characteristic sensuality, and the Renaissance painters Michelangelo and Leonardo da Vinci both depicted her in the arms of the swan, their versions lost or destroyed but their fame undiminished. Every age has found in Leda's story what it wanted to find: a rape, a seduction, a divine mystery, or simply the origin of the most beautiful and most terrible family in myth.