Leonardo is truly a terrible monster—terrible at being a monster that is. No matter how hard he tries, he can't seem to frighten anyone. Determined to succeed, Leonardo sets himself to training and research. Finally, he finds a nervous little boy, and scares the tuna salad out of him! But scaring people isn't quite as satisfying as he thought it would be. Leonardo realizes that he might be a terrible, awful monster—but he could be a really good friend.

One day Sam, the most scaredy-cat kid in the whole world, makes a terrifying discovery. It's not Frankenthaler the monster. It's her friend—Kerry! Kerry, the second most scaredy-cat kid in the whole world, also makes a terrifying discovery. It's not Leonardo the monster. It's his friend—Sam! "AAAAH!" yells Sam. "EEEEK!" yells Kerry. Something has to be done. Something BIG. But what?