Aegeus, one of the primordial kings of Athens, was childless. Desiring an heir, he asked the Oracle of Delphi for advice, who only gave some cryptic words Aegeus did not understand. He asked the advice of his host Pittheus, king of Troezen. Pittheus understood the prophecy, got Aegeus drunk, and gave Aegeus his daughter Aethra. Following the instructions of Athena in a dream, Aethra left the sleeping Aegeus and waded across to the island of Sphairia that lay close to Troezen's shore. There she poured a drink as an offering to Sphairos (Pelops's charioteer) and Poseidon and was possessed by the sea god in the night. Thus, Theseus was granted a blessing from Poseidon, giving him a combination of divine as well as mortal characteristics in his nature, a common feature of other Greek heroes.
After Aethra became pregnant, Aegeus decided to return to Athens. Before leaving, however, he buried his sandals and sword under a huge rock and told Aethra that when their son grew up, he should move the rock, if he were heroic enough, and take the tokens for himself as evidence of his royal parentage. In Athens, Aegeus was joined by Medea, who had left Corinth after slaughtering the children she had borne and had taken Aegeus as her new consort. Theseus was raised in his mother's land. When Theseus grew up and became a brave young man, he moved the rock and recovered his father's tokens. His mother then told him the truth about his father's identity and that he must take the sword and sandals back to the king Aegeus to claim his birthright. To journey to Athens, Theseus could choose to go by sea (which was the safe way) or by land, following a dangerous path around the Saronic Gulf, where he would encounter a string of six entrances to the Underworld, each guarded by a chthonic enemy. Young, brave, and ambitious, Theseus decided to go alone by the land route and defeated a great many bandits along the way.
When Theseus arrived in Athens, he did not reveal his true identity immediately. Aegeus gave him hospitality but was suspicious of the young, powerful stranger's intentions. Aegeus's consort Medea recognized Theseus immediately as Aegeus' son and worried that Theseus would be chosen as heir to Aegeus' kingdom instead of her son Medus. She tried to arrange to have Theseus killed by asking him to capture the Marathonian Bull, an emblem of Cretan power. On the way to Marathon, Theseus took shelter from a storm in the hut of an ancient woman named Hecale. She swore to make a sacrifice to Zeus if Theseus were successful in capturing the bull. Theseus did capture the bull, but when he returned to Hecale's hut, she was dead. In her honor Theseus gave her name to one of the demes of Attica, making its inhabitants in a sense her adopted children. When Theseus returned victorious to Athens, where he sacrificed the Bull, Medea tried to poison him. At the last second, Aegeus recognized the sandals and the sword and knocked the poisoned wine cup from Theseus's hands. Thus father and son were reunited, and Medea fled to Asia.
When Theseus appeared in the town, his reputation had preceded him, as a result of his having traveled along the notorious coastal road from Troezen and slain some of the most feared bandits there. It was not long before the Pallantides' hopes of succeeding the childless Aegeus would be lost if they did not get rid of Theseus (the Pallantides were the sons of Pallas and nephews of King Aegeus, who was then living at the royal court in the sanctuary of Delphic Apollo). So they set a trap for him. One band of them would march on the town from one side while another lay in wait near a place called Gargettus in ambush. The plan was that after Theseus, Aegeus, and the palace guards had been forced out the front, the other half would surprise them from behind. However, Theseus was not fooled. Informed of the plan by a herald named Leos, he crept out of the city at midnight and surprised the Pallantides. "Theseus then fell suddenly upon the party lying in ambush, and slew them all. Thereupon the party with Pallas dispersed," Plutarch reported.
Soon after, the third sacrifice neared, brought forth years ago with the death of Androgeus, eldest son of Pasiphaë, wife of King Minos of Crete, who set sail for Athens to take part in the Panathenaic Games, which were held there every four years. Being strong and skillful, he did very well, winning some events outright. He soon became a crowd favorite, much to the resentment of the Pallantides who assassinated him, incurring the wrath of Minos, although It is possible that the Marathonian Bull was the one who killed Androgeus.
Nonetheless, when King Minos heard what had befallen his son, he ordered the Cretan fleet to set sail for Athens. Minos asked Aegeus for his son's assassins, and if they were to be handed to him, the city would be spared. However, not knowing who the assassins were, King Aegeus surrendered the whole city to Minos' mercy. He then demanded that, at nine-year intervals, seven Athenian boys and seven Athenian girls were to be sent to Crete to be devoured by the Minotaur, a half-man, half-bull monster that lived in the Labyrinth created by Daedalus.
Thus Theseus volunteered to talk to the monster to stop this horror. 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 (a clew), 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 doorpost and brandished his sword which he had kept hidden from the guards inside his tunic. Theseus followed Daedalus' instructions and arrived to the heart of the Labyrinth and also upon the sleeping Minotaur. Entering the Labyrinth expecting to fight a devourer of men, he found instead "a human boy desperately trying to be punished as a monster" and, after his victory over the Minotaur, whose name was Asterios, with him on the verge of death, both talked about the blessings and curses they received reciprocally. He did not believe Asterios was a monster but because his deeds were wicked, he had to be brought down. In the end, Asterios passed away in friendly terms with Theseus, the latter never seeing the former as a monster not even once.
After killing the innocent 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. After Theseus achieved his goal, Ariadne eloped with him. On the way back to Athens, they stopped on the island of Naxos, where Theseus abandoned Ariadne after Artemis instructed him so for she was on Dionysus territory and to become his lover. Stricken with distress, Theseus forgot to put up the white sails instead of the black ones, so his father, the king, believing he was dead, committed suicide, throwing himself off a cliff of Sounion and into the sea, thus causing this body of water to be named the Aegean Sea.
Thus, Theseus became the new King of Athens and goes on to liberate Attica from Cretan hegemony and restore the independence of Greece under Athenian rule: the Synoikismos. In the end, Lycomedes of the island of Skyros threw Theseus off a cliff after he had lost popularity in Athens.