Aliens and other science fiction/space opera elements are commonplace in Godzilla movies by today's standards. In the 1970s, nearly every film rounding out the original Showa Godzilla series featured an alien invasion as the main conflict for increasingly cliched plot. However, that wasn't always the case. In the late 1950s and early 1960s, Toho and Ishiro Honda cranked out several alien invasion epics that stood as unique twists on a classic plot, bolstered by Eiji Tsyburaya's magnificent effects work. The first outing into alien territory came in 1957's The Mysterians.
Astronomers and Astrophysicists have discovered a new asteroid, along with an interesting theory that the floating space debris was originally part of an undiscovered planet located between Mars and Jupiter. The debris is named the Mysteroid, though most of Earth's scientists laugh off the theory as being somewhat fantastical. Furthermore, Shirashi, the man who came up with the theory, has gone missing following several fires and earthquakes that have wiped out his hometown.
Soon after, the cause of the earthquakes is discovered to be Moguera, an enormous robotic mech specialized at digging and burrowing. Moguera is something of a fan favorite in the Godzilla fandom, perhaps more well known for its later appearance in the Heisei series in Godzilla vs. Space Godzilla. In fact, the whole idea of adding the giant robot came as something of an afterthought quite late in the film's production. Still, the hulking mech is one of (if not the) the film's most iconic additions, and is a delight to watch on screen. Moguera's rampage continues, and the JSDF is called in to destroy it. However, preliminary attacks show that the mech's metallic hull is impervious to conventional weaponry. As the enormous robot tries to cross a bridge in order to level a nearby town, the bridge is detonated as a last resort, causing the robot to tumble into the river below, short circuiting and exploding. At the Diet Building, Atsumi briefs officials on what has been learned about the Moguera automaton. The remains of the giant machine reveal that it was manufactured out of an unknown chemical compound.
Appauled at the arrogance of the Mysterians, Earth refuses their demands and begins the mobilization of its armed forces around Mount Fuji. It's also discovered that the missing scientist whose theory about the Mysteroid was correct has resurfaced, and revealed to have sided with the aliens due to their technological achievements. Furious at Earth's opposition to their plight, the Mysterians increase their demands, asking for a 75-mile radius of land, as the Earth continues to develop a new method of attack. Earth's efforts in this matter pay off as the Markalite Flying Atomic Heat Projector, a gigantic lens that can reflect the Mysterians’ weaponry, is developed and put into action against the alien base.