The film concerns the childhood of King Naresuan. Born in 1555, he was taken to Burma as a child hostage; there he became acquainted with sword fighting and became a threat to the Burmese empire
The film begins in 1564, during the Burmese siege of Phitsanulok, the center of the languishing Sukhothai kingdom. Naresuan's father, Maha Thammarachathirat, admits defeat and follows Burmese orders that his two sons, Naresuan (nicknamed Ong Dam Thai: องค์ดำ Black Prince) and Ekathotsarot, be taken hostage and be raised in Pegu (the center of the Hanthawadi kingdom) under the watchful eyes of Bayinnaung, the Burmese king. This creates a rift between Naresuan's father and his mother, Queen Wisutkasat, whose brother is the king of the neighboring Ayutthaya kingdom, as Phitsanulok is now a Burmese vassal state.
Immediately after entering the Burmese palace, Naresuan sees the palace politics and rivalries between himself and Bayinnaung's grandson, Minchit. Naresuan is sent to be educated as a novice monk, by an ethnic Mon Buddhist monk named Khanchong, at a Buddhist monastery outside the palace. There, while wandering the Thai village outside Pegu (made up of Thais displaced by Bayinnaung's expansionist campaigns and subsequent forced relocations to Hanthawadi), he befriends Bunthing, a Thai street child who is later allowed to work as a temple boy. He also befriends Maneechan, a temple girl at the monastery. The monk Khanchong, who had also trained Bayinnaung, teaches Naresuan the skills of war and ethics.