At a local circus, Mie and Kei are assigned to handle a large pink monster they nickname Mon-chan. Feeling remorse for Mon-chan when the ringmaster and trainers physically abuse it, the duo frees it from its cage and run off with it to the forest. While being chased by the ringmaster, Mie, Kei, and Mon-chan end up near Mount Fuji, where they are beamed aboard a UFO. Mon-chan is reunited with its parents as the UFO heads back to their home planet, but Mie and Kei wish to return to Earth. The aliens attempt to beam the duo back to Earth, but a mishap causes them to turn invisible. Upon their return, Mie and Kei use their invisibility to fight crime in Tokyo.