The movie opens on Tien(Tony Jaa) sleeping with a torture collar around his head and arms. Then the guards throw water on his face and wake him up and take the collar off and he falls to the ground. Tien has chains wrapped around his forearms and the guards lift him off the ground and begin to torture him by smacking him with Kendo sticks across his body. The Guards start choking him with Bamboo sticks and Tien jumps in the air and dropkicks the guards and breaks free but his feet are cuffed together and it makes it hard for him to fight. While fighting the guards, one of them catches him off guard and hits him in the face with his stick and he falls to his knees and Lord Rajasena tells the guards to crush his bones. While Tien is tied down on a big rock they start to slam a wooden Battering ram, and hammer into his elbows and knees until they snap, and they finish him off by slaming one into his chest and face, leaving him a bloody mess.
While Lord Rajasena is having bad dreams because of this curse he has over him. Tien’s dead body is tied up outside his palace and Tien’s group of guerilla fighters come and try to get his body but the Crow Ghost (Dan Chupong) shows up and kills them all. Lord Rajasena wakes up from all the noise and begins talking with the Crow. Lord Rajasena wants the Crow Ghost to work for him but Crow Ghost refuses his offer and tells Lord Rajasena that he can remove his curse. The next morning all of the guards are standing around Tien’s body and Lord Rajasena tells them to cut Tien’s head off. Once they go to cut it off a man rides up on a horse and has a paper saying he may take the body with him, and Lord Rajasena gets upset. The man returns Tien’s body to the Kana Khone villagers with Pim and Mhen there. Then the Kana Khone Village gets attacked but they manage to fight off the guards and save Tien’s body. Master Bua who feels bad for letting Tien get hurt shaves his head and becomes a Buddhist monk. Pim starts chipping a rock remaking a sculpture of the ong bak head and brings Tien back to life by reincarnation. Pim Trys to help Tien up while singing a song and Tien gets frustrated and yells at her because he cant walk with his bones broken, so he uses a cane. Tien walks to the edge of a cliff crying and gets ready to jump off and kill his self because he failed to avenge his father but then Mhen talks him out of it. Then Tien with a beard starts trying to get back to normal with the help of Master Bua. Tien starts snaping his bones back into place and begins meditating by a waterfall and dancing with Pim. Meanwhile, Rajasena visits the Crow at his dilapidated temple in an attempt to remove his curse. Beckoning him inside along, the Crow reveals his true motive: to usurp Rajasena and claim his power as King. Rajasena tries to stab the Crow, who merely laughs, throwing him from the temple. Enraged, Rajasena orders his guards to kill the Crow, who dispatches them all with ease. Confronting Rajasena, the Crow launches himself at the King, decapitating him. Rajasena’s severed head curses the Crow with a miserable fate, and the Crow, now King, merely smiles.
While Tien is meditating on a tree, he is ambushed by Lord Rajasena’s guards. Defeating them, Tien races back to the village, only to find it a smoking ruin. Meanwhile, the Crow has taken all the villagers not killed in the fighting (including Pim) to the palace, where he uses them as slaves. Tien appears atop a huge statue, dressed in white and carrying Master Bua’s staff. The Crow, demonstrating his infinite power, unleashes his dark magic and summons an eclipse, causing day to turn to night. In the confusion, Pim reveals herself as Tien’s companion, and is taken to the palace by the Crow’s guards. From the top of the statue, Tien watches helplessly as the Crow cuts Pim’s throat. Enraged, Tien leaps from the statue and begins fighting his way to the Crow. Using swords, spears, and his bare hands, Tien slaughters the guards in a berserk bloodlust. Defeating the horde, Tien approaches the Crow Ghost, covered from head to toe in blood. Laughing, the Crow throws a spear at him, which strikes him through the chest. Tien falls to his knees, defeated, and as he closes his eyes, he remembers what Master Bua told him. Time seems to rewind to Tien atop the mighty statue, and when he opens his eyes, it is revealed to have been merely an illusion caused by the Crow’s magic. Shocked at Tien’s strength, lightning strikes the Crow as the eclipse is dispelled, knocking him from the palace dais and dispatching all the guards. The Crow flees as Tien approaches, engaging him in mortal combat. Picking up a spearhead, the Crow hurls it at Tien, knowing the same move had killed him during the illusion. Tien catches it easily with his preternatural instincts, and throws it aside. Throwing himself at Tien in a blind fury, the Crow is flung from the palace wall, impaling himself on an elephant tusk as he falls to the ground.
The film ends with Tien, Pim, and Mhen praying before the head of Ong Bak along with the rest of the village, as the credits begin to roll.