The Moon Maid was without a doubt Edgar Rice Burroughs' best book. Earth has found piece after decades of war through the complete dominance of the Anglo-Saxon hegemony. America and Britain co-rule the now peaceful planet Earth. Mankind soon turns to exploration of the solar system. The first stop is, of course, the moon. The moon is inhabited by a malevolent race, the Kalkars. Within fifty years the Kalkars completely conquer Planet Earth and all of Mankind. But the Evil Kalkars are not disciplined enough to hold their empire together and Earth falls back to a feudal state. Mankind bides its time until it is strong enough to rise up and fight its oppressors.