Cold, calculating, and deadly, the inhabitants of Planet X were a duplicitous alien race located on a nearby celestial body, visually obscured be the enormous girth of Jupiter. These were a people who mined gold in great abundance, but lacked one key necessity: water. This scarcity prompted the race to attempt the colonization of a new planet, one with a great abundance of the resource they so desperately desired; they needed to conquer Earth! Secretly, the Xiliens, as they were called, landed scouts and advance forces on the planet to pave the way for the next phase of their dastardly plan...

As the primary stages of the invasion were put into effect, the World Space Authority dispatched the P-1 interplanetary vessel to study the newly discovered Planet X. The Controller of the Xilien civilization greeted the astronauts, and explained the dire circumstances that they claimed their planet faced. The Earthlings were made to believe that this small, rocky planet was under the constant threat of King Ghidorah, and permission was requested to operate freely on Earth to procure Godzilla and Rodan. Only these colossal creatures could protect Planet X, the Controller reasoned, and in return for human cooperation, the Xiliens promised the Earthlings a cure for all diseases.

As the nations of the Earth agreed to this proposition, the Xiliens revealed that they had already arrived! Godzilla was lifted from the depths of Lake Myojin, and Rodan was pulled from the cliffs of Washigasawa by the aliens' flying saucers. The monsters were transported deep into space and were released on Planet X, where they fought and repelled King Ghidorah. It seemed as though a great service had been done for these interplanetary neighbors of humankind, and a fond farewell was bid as the astronauts returned to their home world with the medical cure-all. Humankind, however, was deceived!

The secret formula was in fact a device that foretold of the Xiliens' plans for domination. These belligerent beings planned to utilize Godzilla, Rodan, and King Ghidorah, now under their control, to destroy human civilization, unless Earth agreed to become a possession of Planet X. As a plan of defense was being formulated, the invasion began! Luckily, there was one fatal flaw in the plans of these invaders. One of their own kind, who went by the name of Namikawa on Earth, had fallen in love with a human. She passed the secret vulnerability of her race to her lover shortly before her demise. It turned out that the Xiliens' weakness was high amplitude sonic waves! This weakness was exploited when the high-pitched sound was broadcast throughout the region, and the Xiliens writhed in pain. Simultaneously, a device called the a-cycle light ray was used to disintegrate the mind control over the monsters. With their plans for domination thwarted, the Xiliens destroyed their flying saucers and took their own lives. King Ghidorah was soon defeated by Godzilla and Rodan, and the world was rescued from a terrible fate.

Though the invading force was decimated, there were still Xiliens inhabiting Planet X. The long road to diplomatic recovery had just begun...