ninth book in this #1 New York Times bestselling series by Dav Pilkey, the author and illustrator of Dog Man!

George and Harold are behind bars for a crime they didn't commit! But just as they are settling into their new life, they are pulled from prison by a time-traveling tyrant named Tippy Tinkletrousers! Now the boys are taking a trip back in time to the carefree days of kindergarten, when the scariest thing they had to face was not evil mad scientists or alien cafeteria ladies but a sixth-grade bully named Kipper Krupp, the nephew of their clueless school principal. And because George and Harold don't invent Captain Underpants until they're in fourth grade, the clever kindergartners are on their own. Can they beat the bullies with brainpower instead of Wedgie Power?