Since both slept with his mother during the night she conceived, Theseus' father is either Aegeus; the king of Athens, or Poseidon disguised as Aegeus.
Either way, Aegeus left his sandals and sword hidden under a stone; so that when a boy was old enough to lift it, it could go and find his father.
On the road to Athens, Theseus encountered several murders and bandits. One of them, Procrustes, would stretch or cut down guests to fit his iron bed; Sinis would tear people in half using pine trees and Sciron made his victims wash his feet before kicking them off a cliff.
Theseus overcame each of them using their own methods. Though Aegeus did not recognize Theseus when he arrived in Athens, his wife, Medea, did and tried to poison him. The best-known story involving Theseus is that of the Minotaur.
Each year, Athens paid Crete a tribute of seven boys and seven girls, who were fed to the half-human, half-bull Minotaur. Theseus volunteered to be among them but on his arrival in Crete, King Minos' daughter, Ariadne, fell in love with him and gave him a ball of string to help him escape the labyrinth in which the Minotaur lived.
Theseus defeated the monster and then escaped with Ariadne.
However, the tale ends in tragedy. As he was traveling back home, Theseus in his excitement forgot to change the color of his ship's sail from black to white.
Aegeus, reading the black as a sign of his son's death, threw himself into the sea henceforth known as the Aegean.