The Puppet Masters

When a flying saucer reportedly lands in rural Iowa, The Old Man (who runs a secret branch of the Central Intelligence Agency), decides to investigate. He goes in person, accompanied by agents Sam (who is the son of The Old Man) and Jarvis, as well as Dr. Mary Sefton, a NASA specialist in alien biology. They find that aliens have indeed landed and are planning to use their mind-control powers to take over the Earth. The aliens are slug-like creatures, and they are attaching themselves to people's backs, taking control of their victims' nervous systems, and manipulating those people as puppets.