It's the year 480 BC and the greatest army ever gathered in the ancient world is on the march to conquer all of Greece. An irresistible force, they overwhelm all who dare to stand in their way. One man steps forward to stop them, followed by 300 companions. This man is Leonidas. His companions are Spartans. They march to stop the Persian advance and meet their destiny at the narrows known as the Hot Gates.
Twelve-year-old Agis runs away from his boys' group to track the march of the fated 300. He is motivated by the powerful impulse to follow his father into battle and join him fighting the Persians. His presence is discovered too late to send hm back home. Instead, Agis is integrated into the forces and contributes as a look-out to help the Spartans see what is coming before it arrives.