When Aegeus found out Aethra was pregnant, he returned to Athens. He met up with Medea, who had fled Corinth after she had killed Jason's wife. Some stories say she killed Aethra's children with Jason.
Theseus on his way to Athens encountered and defeated the giant Procrustes who lure travelers on to a bed and stretched or shortened his victims to the perfect size for his two beds. Theseus defeated him by trapping him in his own bed, although it is not said whether he straightened or shortened him. He also defeated: Periphetes (the clubber); Sinis (the bender of pines); Crommyonian Sow (a big pig type monster); Sciron (pushed his victims from cliffs); Cercyon (the wrestler). He killed all of them doing the same thing they did to their victims.
When he arrived at Athens, he did not introduce himself immediately, and his father was suspicious of the stranger. However, Medea recognized him and worried that he would be given the crown instead of her son Medus. She challenged him to capture the Cretan Bull. He returned to Athens victorious, but Medea planned to poison him. At the last second the king recognized the sandals, shield and sword that Theseus was wearing as the ones he had left under a boulder for his son. He knocked the poisoned wine out of Theseus' hands, and Medea fled.
Theseus, after King Minos kidnapped some Athenian boys and girls to sacrifice to the Minotaur, decided to descend into the Labyrinth to save them and once and for all kill the Minotaur.
On the third occasion, Theseus volunteered to slay the monster. He took the place of one of the youths and set off with a black sail, promising to his father, Aegeus, that if successful he would return with a white sail. Like the others, Theseus was stripped of his weapons when they sailed. On his arrival in Crete, Ariadne, King Minos' daughter, fell in love with Theseus and, on the advice of Daedalus, gave him a ball of thread. This was so he could find his way out of the Labyrinth. That night, Ariadne escorted Theseus to the Labyrinth, and Theseus promised that if he returned from the Labyrinth he would take Ariadne with him. As soon as Theseus entered the Labyrinth, he tied one end of the ball of string to the door post and brandished his sword which he had kept hidden from the guards inside his tunic. Theseus followed Daedalus' instructions given to Ariadne; go forwards, always down and never left or right. Theseus came to the heart of the Labyrinth and also upon the sleeping Minotaur. The beast awoke and a tremendous fight then occurred. Theseus overpowered the Minotaur with his strength and stabbed the beast in the throat with his sword (according to one scholium, Theseus strangled it on Pindar's Fifth Nemean Ode).
After decapitating the beast, Theseus used the string to escape the Labyrinth and managed to escape with all of the young Athenians and Ariadne as well as her younger sister Phaedra. On the return journey Theseus abandoned Ariadne on the island of Naxos. In other versions of the story, the god Dionysus appeared to Theseus and told him that he had already chosen Ariadne for his bride, and to abandon her on Naxos an island where they stop to get some rest. Ariadne then cursed Theseus to forget to change the black sail to white. Seeing a black sail, Theseus' father Aegeus committed suicide by throwing himself into the sea (hence named Aegean). Theseus and the other Athenian youths returned safely. Theseus returned safely back as a king of heroes.
Theseus and Pirithous went to the underworld in order to capture Persephone. Hades enraged called the Furies to punish Pirithous and enjailed Theseus into his palace doors. While the first hero was freed by Heracles a few months later, Pirithous stayed there.