In the year 1431 in Thailand, Tien (Tony Jaa) is held captive being beaten with wooden staves. On the orders of Lord Rajasena (Sarunyu Wongkrajang), his elbows and knees are snapped. As Lord Rajasena sleeps, Tien's guerilla fighters attempt to free Tien, but Bhuti Sangkha (Dan Chupong) appears and kills them. Lord Rajasena offers to hire Bhuti, but he refuses and gives the offer to remove the curse which has been placed on Rajasena before leaving. Rajasena orders his men to kill Tien, but before this can be carried out, a man arrives with a pardon from the king, indicating that he will take Tien, much to Rajasena's ire. The messenger returns Tien to the Kana Khone villagers. After fending off the village from invaders who are after Tien, Master Bua (Nirut Sirijanya) feels guilty over Tien's imprisonment and becomes a Buddhist monk. Pim (Primrata Det-Udom) heals Tien to life, but finds that Tien is still crippled from his beatings. Tien then embarks on a rehabilitation regimen with the help of Master Bua.
Rajasena visits Bhuti at his temple to remove his curse, but Bhuti reveals his true motive of usurping Rajasena and becoming the new king. After a battle, Bhuti decapitates Rajasena, but his severed head curses Bhuti. After meditating, Tien returns to his village to find it in ruins, and the surviving villagers kidnapped and enslaved by Bhuti. Bhuti uses his magic to summon an eclipse. When Pim reveals herself as Tien's companion, she is taken to Bhuti's palace, where she is killed. Tien witnesses this killing from a statue and fights his way through the guards before confronting Bhuti, who launches a spear at Tien's chest. As he falls to his knees, defeated, he remembers Bua's words, and finds himself again atop the statue. Overcoming Bhuti's illusion, lightning strikes and Bhuti's eclipse magic is dispelled. Bhuti attempts to escape but is confronted by Tien. Bhuti attempts again to throw a spear at Tien, who catches and throws it aside. On the royal ledge above the arena, Tien's upright finger tips hold Bhuti aloft by his chin. Suddenly an elephant's trunk butts the doors below the ledge, causing Bhuti to fall from Tien's grasp. Bhuti falls, over the elephant's tusks. The camera pans as we watch Bhuti dying on the ground, pierced by the elephant's broken tusk. As he breaths his last breath, the elephant, now resembling the one tusk Ganesha, raises his head in a victorious trumpet. Beginning life anew, with good having triumphed over evil, the final scene opens as Tien, Pim and the remaining villagers, pray before the statue of Ong Bak.