Aliens from the mysterious Planet X, which resides on the dark side of of Jupiter, come to Earth asking its people to help them save their world from the dreaded King Ghidrah by letting them "borrow" Godzilla and Rodan. The aliens are actually planning to use the three monsters to take over our planet.

Earth astronauts Glenn and Fuji are sent to Planet X to establish friendly ties with the ruler of that planet, The Controller. He promises Earth the formula for a universal panacea in return for the loan of two Earth monsters, Godzilla and Rodan, to combat the fearsome Ghidrah (Monster Zero) which is terrorizing the people. Returning home to Japan, Glenn becomes romantically involved with Namikawa, an enemy spy posing as a toy manufacturer's representative. Godzilla and Rodan, released from their lairs, are transported by huge space bubbles to Planet X. Glenn, Fuji, and a doctor follow in a spaceship. They soon discover that the tape which is supposed to contain the medical formula to end disease is actually an ultimatum for Earth's immediate surrender. Refusal would mean the Earth's destruction by the three monsters which are controlled by Planet X's computers. Namikawa is killed, and a toy created by Teri, the fiancé of Fuji's sister, is discovered to emit a sound that severs the light waves by which the creatures are controlled. The invaders are destroyed in a space battle with cannons and laser beams; Ghidrah defeats the Earth monsters and flies away.