Lonely orphan Simon (pronounced "Shi-mon" in Japanese and "SEE-mon" in the dub) lives in an underground village, digging for ancient artifacts and eating pigmoles. His self-appointed big brother, Kamina, is a hot-headed idiot gang leader/delinquent who dreams of going to the "Surface" like his father, despite the village elder's insistence that there's no such thing.
Then Simon discovers a strange drill-shaped key and, later on, a pint-sized mecha that seems to respond to the key.
Taking his "brother" to see it, they're interrupted by, in turn, the elder, the roof collapsing, a massive bestial robotic head (which Kamina faces off against with just a katana), and Yoko, a woman with a very big gun and very little clothing firing at said beast-mecha — a "Gunmen".*
Thus begins a rollercoaster ride of Fanservice, over-the-top fight scenes between mecha, hot-bloodedness, epic sunglasses, massive Badassery, and mind-blowing heroism. Great fun, and doesn't take itself seriously at all. Don't try reading any science into it, common theory (and near Word Of God) holds that the Universe is actually governed by Rule Of Cool and Rule of Funny. On Mohs Scale of Science Fiction Hardness, it ranks as one of the softest.
