Evil, slug-like aliens land on Earth. They attach themselves to people and take over their minds. After a while, everyone has to walk around completely naked, so that you can spot the ones who have a slug attached.
I just can't understand why Paul Verhoeven didn't film this. He could have combined
Starship Troopers and
Showgirls into one movie, and both he and the rest of the world would have been happier. My guess is that he discovered the book too late, and has been kicking himself ever since.