You can say anything in a letter -- as long as you don't mail it!
It was bad enough when Kerby got a box of stationery for his birthday. But then Kerby's mom makes him use it to write thank-you letters. Yecch! Kerby doesn't think this is fair. There are much better things to do with a box of fancy paper -- like writing nasty notes to your enemies!
It feels great telling off grouchy Mrs. Pembroke and the neighborhood bully on a piece of paper. But the joke's on Kerby when the letters are accidentally mailed!
If anybody reads those letters, Kerby's in BIG trouble. He just has to get them back -- any way he can!