In feudal Korea, toward the end of the Goryeo dynasty, a king's iron rule leaves the peasantry starving and powerless. Takse, Gwon Ri, the land's finest blacksmith, is imprisoned and starved for defending his people. Before he dies, he carves a tiny rice figurine of a monster and pleads with the gods to turn his creation into a living guardian for the rebels. The figure finds its first home with his daughter Ami, Chang Son Hui. When Ami accidentally wounds herself while sewing, the figure touches her blood and comes to life. The creature, Pulgasari, Kenpachirō Satsuma, emerges and, drawing from the myth of an iron-eater, bonds with Ami and grows into a formidable metal-eating guardian.
Pulgasari forms a close bond with Ami and grows ever stronger after feeding on a farmer's tools. With hunger as its fuel, the monster becomes a towering protector, and the peasants rally around this unlikely ally as they plan to overthrow the monarchy. General Fuan, Riyonun Ri, commands the royal army and faces the uneasy alliance between people and monster. The rebels capture Ami and threaten to kill her unless Pulgasari enters a massive cage they have prepared. The creature lets itself be trapped to save Ami and is set ablaze, but remains unharmed.
Undeterred, the rebels storm the region's Governor’s palace and strike a decisive blow. They kill the governor, and the king begins to grasp that a rebellion is spreading. Pulgasari continues to win battles by devouring the metal weapons of the royal army, turning its fearsome appetite into an alarming advantage for the rebels. The royal forces respond by burying Pulgasari, capturing Inde, Ham Gi Sop, the rebellion's leader whom Ami is betrothed to, and threatening Ami with death if the rebels do not surrender.
Ami escapes and, with resolve, revives Pulgasari by pouring some of her blood onto the burial site. The monster rises again, more immense than before, and launches a new assault on the king's palace, bringing it to ruin and ending the king's rule. Yet triumph comes with a grim realization: Pulgasari's hunger now stretches beyond mere defense, as it begins to consume the weapons and tools of peasants and soldiers alike. The people start to fear that their savior may become a new oppressor.
Aware of this dangerous turn, Ami makes a final, selfless choice. She hides inside a large bell that Pulgasari discovers and promptly eats. The monster emits a cry of anguish as Ami's presence within its body causes it to turn to stone and crumble, killing both of them but saving the people from a future ruled by iron and fear.
