Carl Sims, a young virologist, discovers a plot hatched by a group of international scientists to cull, in a matter of weeks, two-thirds of the world's population - some 4.5 billion people, by releasing a deadly virus that kills two-thirds of those it infects. Their goal is to reduce Earth's population from an unsustainable seven billion to two billion. What is he to do? Try to stop the conspiracy, or join it?