Yellowstone National Park ranger Tucker picks up a frantic distress signal: a tour group has vanished without a trace in the park's treacherous geothermal hot spring basin.
He follows the trail to a billowing steam fissure, where he comes face to face with a nightmare: a six-legged, molten-skinned creature that speaks with the voices of the missing tourists.
Tucker empties his pistol into the monster, but the bullets simply melt on contact with its superheated skin. He leaps into a frigid glacial stream to escape, and the creature recoils violently, hissing in agony from the cold. His report is dismissed as a hallucination by his superiors, but he refuses to back down and teams up with Sloane, a sharp-witted geologist who has been tracking the park's unusual seismic activity.
Together, they uncover terrifying evidence: the geothermal system is undergoing a catastrophic shift, and the creatures are ancient subterranean lifeforms that have been awakened by the rising heat. Their only weakness is extreme cold.
The pair are soon hunted through the dense pine forest by a pack of the lava mimics. They fight back using cryogenic fire extinguishers stolen from a park maintenance shed, but their supplies run dry within minutes. With no other options, Tucker hatches a desperate plan: divert the entire flow of a nearby glacial river into the main fissure network to freeze the entire colony solid. The only problem? The control valve is perched directly above the creatures' underground nest.
They descend into the heart of the geothermal zone, using prearranged code words to avoid falling for the monsters' flawless vocal impersonations. When they finally confront the colossal matriarch creature, Sloane fights her way to the valve and slams it open. A raging torrent of icy glacial water surges into the fissures, encasing the lava mimics in thick sheets of ice.
But as they stumble back to the surface, a deep, bone-rattling vibration rolls through the ground. Seismic readings confirm the worst: the vast majority of the underground colony has already escaped the freeze, and is now burrowing at incredible speed directly toward the unsuspecting nearby town.
