The Hero's Journey is a framework that scholar Joseph Campbell came up with that many myths and stories follow. Many storytellers and story-readers find it a useful way to look at a tale. (That's actually putting it lightly. Some people are straight-up obsessed.) Chris Vogler adapted Campbell's 17 stages of a hero's journey, which many screenwriters use while making movies. Vogler condensed Campbell's 17 stages down to 12, which is what we're using. Check out a general explanation of the 12 stages.
The story of Theseus doesn't fit perfectly into the Hero's Journey structure, but we're giving it a shot. As the gross old saying goes, there's more than one way to skin a cat.
Theseus lives a happy and peaceful life as the Prince of the sleepy city of Troezen.
Call to Adventure
Aethra, Theseus's mother, tells him that he's the son of Aegeus, the King Athens. She reveals that his father's sword and sandals are hidden under a rock. In order to claim his birthright, Theseus must lift the rock, take the sword and sandals, and join his father in Athens.
Refusal of the Call
Theseus doesn't actually refuse the call at all. He's eager to get the adventure underway and find his destiny.
Meeting of the Mentor
Theseus kind of skips this step, too. Unless, you count his mom, Aethra, who tells him how to fulfill his destiny. She doesn't really help him out as much as most mentors would, though.
Crossing the Threshold
The young hero lifts the rock and claims his father's sword and sandals. He's now ready to take the first step into adventure.
Tests, Allies, Enemies
To prove himself, Theseus chooses to travel to Athens by land, which is mad dangerous. Along the way, he kills six bad guys, most of the time using their own methods against them.
Approach to the Inmost Cave
The biggest danger that Theseus faces actually ends up being his stepmother, Medea. When he arrives in Athens, she convinces his father, King Aegeus, that the young hero is a threat. Aegeus doesn't yet know that Theseus is his son, so he agrees to sending the young hero off to fight the Marathonian Bull, hoping that Theseus will do everybody a favor and die.
Using all his strength and skill, Theseus conquers the giant bull and returns to Athens in triumph.
Reward (Seizing the Sword)
Everybody in Athens thinks Theseus is totally awesome for defeating the bull and the six bad guys on the way to Athens.
The Road Back
A huge banquet is held in Theseus's "honor." Theseus prepares to reveal himself to his father, but...
Theseus comes super close to death, though, when Medea and Aegeus give him poison wine.
Return with the Elixir
Luckily, Aegeus recognizes Theseus's sword just in time. The King knocks the poison wine out of Theseus's hand and embraces the young man as his son and heir.