Earth is invaded by a race of creatures that have the ability to control a human being like a puppet on a string; hence they are called “Puppeteers” or “Puppet Masters.” Physically, the aliens resemble puddles of mercury, hence the other name for them: “The Blobs.” The aliens are capable of reproducing by dividing in two, though this puts a strain on their host. Because of this, and because a person “hosting” a Puppeteer is very difficult to distinguish from a normal person, the invasion spreads at an exponential rate.
Mike, a 24 year old man who suffers from depression, is “converted” while having lunch at a Burger King. Unlike most hosts, he finds possession to be a positive experience largely because the presence of the alien seems to cure his depression. Alice, a woman with bipolar disorder (manic/depressive), has a similar experience.
Scientists in the US government discover that the aliens are very sensitive to encephalitis, a disease that infects the central nervous system. The condition is spread from the host to the Puppeteer, where it proves to be almost 100% fatal. The US unleashes a virus that effectively wipes out the alien presence, though it also results in about 50 million human deaths worldwide.
Mike finds that, without the presence of the Puppeteer, he sinks back into depression. As a result, he volunteers with the Planetary Defense agency to be a host to an alien, so that humanity can gather additional information about the Puppeteers. PD is an organization that the world’s governments have formed to combat the alien menace in the wake of the invasion.
Once again, the alien’s presence causes Mike’s depression to go into remission. The Puppeteer also informs Mike that the remaining aliens are likely to try bombarding the Earth with asteroids before invading again. Mike’s efforts to alert PD of the threat are met with scorn however, causing Mike to try to persuade an officer named Tom to help him. Tom believes Mike and the two of them escape from PD custody in an effort to try and save Human civilization.
Mike recruits other people with conditions similar to his own and meets Alice again. “The Guided,” as they like to call themselves, resolve to hijack an alien spacecraft and then to use it to stop the aliens from bombarding the Earth. The first part of the plan is successful; the team manages to capture a scout craft that was attempting to reconnoiter the situation on Earth. It is at this point that we pick up the story.