Evil moon men have landed on the earth and have formed a political alliance with the evil Queen of Samar. The moon men have been demanding human sacrifices, in this case the citizens of Samar. Hercules is called in to put an end to this stuff.
This is the usual standard Hercules plot, he pulls out a tree and clobbers guys with it, he bends bars, he fights a monster named "Monster", he throws a lot of guys around, etc. This is silly Italian junk with lots of closeups of our hero's well oiled and manly chest glistening, glistening, glistening as he performs his manly feats of strength. It's all very homoerotic and quaint.
Hercules vs. the Moon Men wraps up with our hero fighting some stuntmen dressed as moon alien rock men. The alien rock men lumber around the set as Hercules tosses them around like they were a salad that needed mixing. Meanwhile, stock shots of volcanoes and hurricanes are inserted to signify the end of the world or something.
Not to worry it all ends happily with our hero riding off into the sunset to help the next kingdom in need of his services.