Twelve-year old Agis runs away from his training barracks to follow the 300 Spartan warriors that King Leonidas is leading north to stop the Persian advance.  One day, he is captured suspected of being a spy.  They march him back to camp and present the boy to Leonidas.       
Once the king is satisfied the boy is freeborn and not an escaped slave, he summons his companions to pass judgment upon him.  Strict obedience is a virtue among the Spartans, and breaking rules is severely punished.  When the Spartan warriors hear of the boy’s resolve to join them to fight the Persians, they approve of his high spirits.  Leonidas proclaims the lenient punishment of a single lash, to remind him that a Spartan never turns his back in battle.           One man steps forward to deliver the punishment.  It is the one man Agis has come to see, yet the one man he dreads being disgraced before: his father.  His father, Nikandros, delivers the lashing.  Leonidas, guessing their relationship, tells Nikandros to take the boy back to his companions and look after him. 
They encounter another troop of warriors from the town of Thespiae, also on route to Thermopylae.  Leonidas welcomes them, but points out that the Thespian hoplites all make their living as farmers, stone-carvers, weavers and the like, yet every one of the Spartans is trained to be a warrior.  Leonidas declares that his troops will take on the brunt of the fighting, and will keep the other Greeks in reserve.          
They reach the Hot Gates, a narrow pass between the mountains and the cliffs overlooking the waters of the gulf.  It is a “choke points” of Greece, a narrow passage around the mountains that anyone traveling south must pass through.  The Persian forces will have to pass here, and here  the Greeks will make their stand.  The name of the pass is due to the hot springs that flow there.
They find the remnants of a wall, and they work to rebuild it.  Soon, scouts approach the pass, looking what resistance they will find.  Leonidas banters with them.
Although his father wants him safely away from the battle, Leonidas places Agis in a protected lookout to watch the road ahead and give warning.  For two days the Spartans hold the pass.  The Persian king throws his personal elite forces, the Immortals, against them, but to no avail.           
 
Then word comes that the Greeks have been betrayed.  A traitor has revealed a goat path over the mountain and the Persians are already crossing to attack the Greeks from behind.  Leonidas sends all the Greek reserves away, but the Spartans remain.  He orders Agis to lead a blinded hoplite back to Sparta.  They are to report that those who remained have been true to the common law of Sparta, never to retreat from battle.  With a heavy heart, Agis obeys this last order from his king, departs from his father, and leads the wounded hoplite back to Sparta.