The Puppet Masters is a novel by Robert A. Heinlein about an Alien Invasion, first published in 1951. This was the only novel Heinlein published on the subject and he cemented most of the tropes usually associated with invasion stories. Secret agents Sam, Mary and "The Old Man" fight a fantastical battle against parasitic aliens who are bent on assimilating all of humanity and taking control of Earth. Danger is everywhere when anyone around you could be a host.