Dr. Eustus Riko, a discredited physicist, created a machine which allows living beings to "Dissolve" into the parallel Dimension 2-K-31. His first success was the transfer of a large ape. However the matter displaced from the human plane forced a creature from the other side to cross over.
The creature happened to be a living weapon, engineered by the extra-dimensional scientist, Katu, to end his society's war. It possessed complete control over its form, changing size, shape, and substance at will.
It ended up appearing in a Korean war zone. The first to spot the Creature was a Korean boy who mistook it for a dragon and alerted a troop of American soldiers to capture it. They fired upon the monster only to find gunfire completely ineffectual against its thick, slimy hide. It fled into the battlefront. As another battalion joined the chase, it crawled into a shell hole and transformed into one of its attackers. As the soldiers debated which was the real sergeant, the creature broke loose, running into enemy territory.
There, the creature killed the Korean commander but became trapped within a tank. The Americans seized the territory, which had been left deserted after the Creature's rampage. As the story comes to an end, the tank is being transported to the states to be dismantled for parts, while the Americans are completely unaware it holds the monster they were so helpless to stop.