Tomorrow, totally unexpectedly, a spaceship arrives from another galaxy. It is filled with males of human shape but of a previously unimaginable, and decidedly unpleasant, skin colour. These strange beings have developed technology which is all but incomprehensible to us. Communication is achieved by extrasensory perception.
People can travel at the speed of light.
Law and order is based on a value system which we find bizarre, irrational and basically evil. Food production is achieved by reprocessing soil and rocks in huge factories. They can neutralise nuclear weapons and their own weaponry is so powerful that, as they inform us, if we don't obey their every instruction, they will use their selective ray gun which can destroy humans and leave all other species intact. At first they don't interfere in human affairs. They have brought their own labour force and, within weeks, they are building factories and installing scoopers and earthmoving equipment.