What does Arnold Mayhew get when he and his friend accidentally combine a stolen test tube full humongosaurus dinosaur growth hormone with a sack of ordinary Idaho potatoes? They get some pretty mean potatoes--the size of ten-story buildings!

Of course, Arnold's sister Courtney thinks the whole thing is a great science fair project. And his best friend Max Beekly thinks it's a hilarious practical joke. But there's nothing funny about these potatoes. They've taken over the subway system, they've invaded the baseball stadium, and they're out to destroy the city.

These potatoes won't be fried, they won't be mashed and they must be stopped...before they scallop the whole town!