The early parts of Theseus' life contain the most mythical elements. Plutarch states that Theseus was "said" to have had a god as a father, but really Theseus was the bastard son of the King of Athens. Upon coming of age, Theseus learns the truth of his parentage and travels to Athens to claim his patrimony. Along the way, he battles against and kills a number of beasts and men who had been terrorizing the Greek countryside, pacifying a once wild land and earning the gratitude of the Greek people. These tasks are obviously patterned after the labors of Hercules, and Plutarch describes Theseus as an admirer of Hercules (who was a generation older). They also foreshadow Theseus' career, which involved repeated instances of his destroying the wild barbaric elements of the world to secure peace and civilization for the people of Greece.
Upon reaching Athens, the King - Theseus' father - recognizes Theseus as his son because Theseus is carrying a sword the King left behind with Theseus' mother. Welcomed into the King's household, Theseus learns that Athens has been paying tribute to King Minos in the form of 7 young men and 7 young women every 9 years. This section of Theseus' life (he travels to Crete, enters, the labyrinth, and kills the Minotaur, and frees Athens from its thrall to Minos) is a familiar story from Greek myth. Plutarch allows that this version of the story IS pure myth, but then puts forth several plausible alternative stories that were known to the ancient Mediterranean world, and which may have provided the historic germ from which the Theseus myth was ultimately drawn.
Upon returning to Athens, Theseus becomes King of Athens. At that time, Athens was simply a city and nothing more. Theseus gathers the surrounding towns and countryside around Athens' political orbit, and establishes the democratic Athenian city-state that would become the birthplace of civilization. Theseus reassures the populace by giving up all of his power except his power to protect Athens from invasion, and enforce the laws established by the Athenian people. This is really Theseus' most impressive achievement, but Plutarch does not really dwell on it all that much, seeming to prefer tales from Theseus' adventures in battling and exploring.
Eventually, Theseus is brought down by a woman, and not just any woman; Theseus kidnaps the teen aged Helen (later of "of Troy" fame) from her father's house. Although there are hints of a "ravishment" (hubba hubba),Plutarch primly assures us that Theseus intended to "wait until Helen was of a marriageable age." Sure. This causes a war, with Castor & Pollux leading an army to Athens to demand the return of Helen. Some of Athens' former political leaders who lost power after the arrival of Theseus take advantage of the situation to reclaim their lost power. This section of the story could have been written anytime in the last 2500 years:
"Menestheus ... the first man that is recorded to have affected popularity and ingratiated himself with the multitude, stirred up and exasperated the most eminent men of the city, who had long bore a secret grudge to Theseus, conceiving that he had robbed them of their several little kingdoms nd lordships, and having pent them up all in one city, was using them as his subjects and slaves. He put also the meaner people into commotion, telling them that deluded with a mere dream of liberty, though indeed they were deprived of both that and their proper homes and religious uses."
Plutarch does not mention whether Theseus "shredded the Constitution." Theseus is later returned to Athens by Hercules himself, but by then the bloom was off the rose:
"Wishing immediately to resume the first place in the commonwealth, and manage the state as before, he soon found himself involved in factions and troubles; those who had long hated him had now added to their hatred contempt; and the multitude of the people were so genuinely corrupted that ...they expected to be flattered into their duty."
Theseus leaves in disgust, and eventually dies in obscurity. After his death, the Athenians regret their dismissal of their great hero and move Heaven and Earth to discover find his remains and return them to Athens for a proper burial, repeating throughout the Ancient Era, "Nothing Without Theseus."
Theseus' life corresponded with what can only be described as Greece's Age of Heroes. His life spanned the period just before the Trojan War. He met or crossed paths with Hercules, Jason, Castor, and Pollux, and many other smaller heroes, among others. He fought a major war against the Amazons. He killed the Minotaur, and freed Athens from the thrall of King Minos of Crete. He then served as a sort of "Father of His Country" as to Athens, while also going on a series of adventures with the heroes of his day that effectively civilized the Eastern Mediterranean and expanded the Ancient's knowledge of the world. While Theseus was a man of violence, his violence is used as a civilizing tool whereby he brings peace to a chaotic land, and giving rise to the conditions and institutions that would allow Greece to thrive.