There once came a time where the Athenians where at war with Minos, the King of Crete, because they had killed his son Androgeos. The punishment was to send nine boys and nine girls to Crete every year in order to be devoured by the monstrous Minotaur.
In order to put an end to the Athenians grief, Theseus decided to go to Crete and slay the Minotaur. The Minotaur was imprisoned inside the labyrinth- a confusing, complex building constructed by Daedalus. Once somebody entered the labyrinth, it was impossible for him to leave and he was devoured by the monster.
However, King Minos' daughter and princess of Crete Ariadne fell in love with Theseus at first sight and promised to help him beat the Minotaur. She therefore gave Theseus a spool of thread that he let unwind through the labyrinth so that he wouldn't lose his path.
Theseus managed to kill the Minotaur and, using this thread, to get out of the labyrinth and to finally escape from Crete.