After Athens loses a war with Crete, King Minos imposes a recurring tribute: every nine years, fourteen Athenian youths, seven young men and seven young women, must be sent to Crete to be devoured by the Minotaur, a monster with a bull's head on a man's body born to Minos's wife Pasiphae after a curse from Poseidon, and confined within an elaborate labyrinth built by the craftsman Daedalus specifically so that anyone who enters cannot find their way back out. When the third tribute comes due, Prince Theseus volunteers to go among the intended victims, over his father King Aegeus's objections, intending to kill the Minotaur and end the tribute for good.
Arriving in Crete, Theseus wins the love of Minos's daughter Ariadne, who secretly gives him a ball of thread to unwind as he enters the labyrinth, so that he can retrace his path back out after finding and killing the Minotaur. The plan succeeds: Theseus kills the Minotaur, follows the thread back to the entrance, and escapes Crete with Ariadne and the other freed Athenian youths.
The Minotaur's existence was itself a punishment: Minos had once asked Poseidon for a magnificent bull as a sign of favor, promising to sacrifice it back to the god, but kept the bull for himself because of its beauty, and Poseidon retaliated by making Minos's wife Pasiphae fall in love with it, a union Daedalus made possible by building her a hollow wooden cow to hide inside, and from which the Minotaur was born nine months later.