Sam, Mary, and The Old Man--the head of the clandestine
national security agency called the Section--travel to Iowa to investigate a
report involving a flying saucer as well as the disappearance of six agents
previously on the case. Learning that the city is being overrun by slugs, they
are disheartened to learn that they can't convince the US President to
officially declare an emergency.

The three are attacked but manage to escape. However, a slug
is able to escape with them. The situation becomes more dire when Sam gets
taken over and the slug starts using Sam's vast skills and knowledge to
facilitate the invasion.

Is this the beginning of the end for the human race?