Tyndareus' wife, Leda, was seduced by Zeus, who disguised himself as a swan. She laid two eggs, each producing two children. From one egg, Pollux and Helen were the children of Zeus; from the other, Castor and Clytemnestra were the children of Tyndareus.
Tyndareus had a brother named Hippocoon, who seized power and exiled Tyndareus. He was reinstated by Heracles, who killed Hippocoon and his son, Lycon.
When Thyestes seized control in Mycenae, two exiled princes, Agamemnon and Menelaus came to Sparta. Tyndareus received them and Agamemnon married Clytemnestra. Helen, however, had many more suitors for she was the most beautiful woman in the world.
When it was time for her to marry, many Greek kings and princes came to seek her hand or sent emissaries to do so on their behalf. Among the contenders were Odysseus, Menetheus, Ajax the great, Patroclus and Idomeneus, but the favourite was Menelaus who, according to some sources, did not come in person but was represented by his brother Agamemnon. All but Odysseus brought many and rich gifts with them.
Tyndareus would accept none of the gifts, nor would he send any of the suitors away for fear of offending them and giving grounds for a quarrel. Odysseus promised to solve the problem in a satisfactory manner if Tyndareus would support him in his courting of Penelope, the daughter of Icarius. Tyndareus readily agreed and Odysseus proposed that, before the decision was made, all the suitors should swear a most solemn oath to defend the chosen husband against whoever should quarrel with the chosen one. This stratagem succeeded and Helen and Menelaus were married. Following Tyndareus' death, Menelaus became king of Sparta because the only male heirs, Castor and Polydeuces had died and ascended to Mt. Olympus.
Some years later, Paris, a Trojan prince came to Sparta to marry Helen, whom he had been promised by Aphrodite. Helen fell in love with him and left willingly, leaving behind Menelaus and Hermione, their nine-year-old daughter.
(According to the Iliad, however, Helen was still unaware of her brothers' deaths in the tenth year of the Trojan War, since during Book III she looks for them among the Greek host and is surprised not to see them. This suggests that Castor and Polydeuces died sometime after Helen's departure for Troy but before the war itself.)
Menelaus' attempts to retrieve Helen, eventually successful, caused the Trojan War.
Alternative: Tyndareos