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 neighborhood bully on a piece of paper. But the joke's on Kerby when the letters are accidentally mailed!
If anybody reads those letter, Kerby's in BIG trouble. He just has to get them back--any way he can!