The film begins with a giant fissure destorying an entire village. This leads to an investigation whereby the source is discovered to be Moguera, a giant robot, who is then destroyed by the military. The remains are analyzed and discovered to be of alien origin. Shortly after, an alien race known as the Mysterians arrive, declaring they have taken some Earth women captive and that they demand both land and the right to marry women of Earth.

References:
https://en.wikipedia.org/wiki/The_Mysterians