Theseus was the son of Aegeus, king of Athens. At the time of the story, there was great rivalry between Crete and Athens, but because Crete was more powerful, the king of Athens had to send 7 young men, and 7 young women every year to Crete to be slaughtered by the gruesome Minotaur (the half man/half bull creation born from the queen).
Theseus (Aegeus’s son) volunteers to go to Crete to try and kill the Minotaur. His father did not want him to go, and so he told Theseus to put up a white sail if he had succeeded, and a black one if he failed.
On his arrival, the king of Crete’s daughter (called Ariadne) falls in love with him and decides to help him. She gives him a ball of string to find his way around the maze, and a dagger/ sword with which to kill the beast.
Theseus kills the Minotaur, and escapes the maze by following the ball of yarn back to the entrance. He escapes the island with the daughter.
On the way back, they stop off at Naxos. Theseus was scared of what the Athenians would say if they saw him married to one of the enemy, and so he leaves her behind sleeping under a tree. When she awakes she is very upset, but the god Dionysus comes down to marry her.
Theseus is either upset at this episode, or so happy to be returning home, that he forgets to change his sail from black to white. His father, Aegeus is watching from the cliffs, and when he sees these sails come into view, he throws himself off into the sea. This is now called the Aegean Sea.