Captain Underpants and the Preposterous Plight of the Purple Potty Peo

George, Harold, Sulu, and Crackers end up in an alternate universe where the whole world is the opposite of their normal world. Good is bad, smart is dumb, and heroes are villains. For example, Melvin Sneedly is struggling to comprehend a simple children's book and Mr. Krupp is nice and has a sense of humor. There they meet Captain Underpants's evil twin, Captain Blunderpants (Nice Mr. Krupp's super-villain identity) and an Evil George and an Evil Harold. Sulu and Crackers are stolen by Evil George and Evil Harold and hypnotized to be evil. Sulu becomes evil and attacks the good George and Harold. Crackers saves the boys. The heroes zap back to the real world, but bring the Mr. Krupp of the alternate universe, Sulu, and Evil George and Harold with them. Evil George and Harold transform their Mr. Krupp into Captain Blunderpants by getting water on his head. Evil George and Harold transform Sulu into a giant monster, who is defeated by Captain Underpants. Captain Blunderpants is transformed back into Nice Mr. Krupp by the snap of the finger, but a rain storm turns him back into Captain Blunderpants. Captain Underpants is turned back into mean Mr. Krupp and goes home. Great Granny Girdle and Boxer Boy (George's great-grandmother and Harold's grandfather), made super-heroes when they drink super power juice, defeat Captain Blunderpants. Sulu is transformed good again and Captain Blunderpants and the Evil George and Harold are taken back to their world. Tippy Tinkletrousers (Professor Poopypants changed his name at the end of the fourth book) comes and chases George and Harold. The book ends with George yelling 'Oh no!' and Harold yelling 'Here we go again!'