They had come to save us but the price was very high! They had come to save the earth. But the price of salvation was that the population of earth had to be reduced to no more than one billion people in 20 years. The Visitors almost immediately began an extermination program to achieve this goal. In an attempt to stop the killing, their spaceship was attacked with nuclear weapons and laser beams, but to no avail. Eventually, Dr. Dunston was able to go to the Visitors' ship and negotiate a longer time to carry out their mandate, but the killing would still continue. The earth, however, was indeed recovering, which was the Visitors' goal.