Captain Underpants and the Preposterous Plight of the Purple Potty People

Captain Underpants and the Preposterous Plight of the Purple Potty People is the eighth book in the "Captain Underpants" series by Dav Pilkey. It was published on August 15, 2006, two years and eleven months after the previous book. This was the last Captain Underpants book to be published for 6 years, until Captain Underpants and the Terrifying Re-Turn of Tippy Tinkletrousers was published in 2012.
Plot summary.
George Beard, Harold Hutchins, Sulu, and Crackers have now ended up in an alternate universe where the whole world is the opposite of their normal world (instead of being perched up in a tree in the Cretacious Period of the Mesozoic Era). 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 humour (Like Captain Underpants does). After they realize these events taking place and then some, there they see evil versions of George and Harold (George and Harold's evil counterparts) and they learn through one of their evil twins' comics that they had turned their principal into an evil supervillain named Captain Blunderpants. Sulu and Crackers are kidnapped by Evil George and Evil Harold. They are hypnotized to be evil. (Crackers isn't hypnotized because she is female) Sulu becomes evil and attacks Good George and Good Harold but Crackers on the other hand, saves them (which Good George and Good Harold can't understand why). 
The heroes finally get back to their normal dimension of the multiverse, but end up unintentionally bringing Nice Mr. Krupp, Sulu, and Evil George and Evil Harold with them. Evil George and Evil Harold transform Nice Mr. Krupp into Captain Blunderpants by getting water on his head and the 4 began to wreak havoc into the town of Piqua. Good George and Good Harold take the 3D Hypno-Ring so that they could dehypnotize Sulu and take the Extra-Strength Super Power Juice just in case. However, Mr. Beard stops Good George and Good Harold from leaving the house and forces them to come inside as it is Grandparent's Day and Good George and Good Harold have to eat dinner with their grandparents at Good George's house. While Good George and Good Harold try to explain to Mr. Beard they need to leave, George and Harold's grandparents take the remainder of the Extra-Strength Super Power Juice and drink it.
Elsewhere, Evil George and Evil Harold find Good George and Good Harold's treehouse, rummage through their personal belongings and find the "Goosy-Grow 4000". They then use it to transform Sulu into a giant monster named Hamsterzilla. Hamsterzilla charges at Good George and Good Harold, but Crackers flies in and Good George and Good Harold flee from their home flying over on Crackers' back, only to be chased by Evil Harold, Evil George and Captain Blunderpants. The Good George and Harold decide that they have to drink the super power juice to fight back, but unfortunately, they realize it is somehow empty despite there being a third amount of juice earlier. Since there was no more of the Super Power Juice left, Good George explains that there is only one plan left they can resort to now.
The duo and Crackers manage to escape the evil duo that was following them. They go to Nasty Mr. Krupp's house (which Good George and Good Harold covered in toilet paper at an earlier point) and knock on his door. Right after Nasty Mr. Krupp answers, Good George and Good Harold quickly snap their fingers to turn Nasty Mr. Krupp into Captain Underpants. The latter then defeats Hamsterzilla. The evil trio notice this after robbing a bank, so Underpants now must fight his evil counterpart Blunderpants. However before the epic battle begins, Captain Blunderpants is transformed back into Nice Mr. Krupp by the snap of Good George's finger. The duo reveal that they didn't forget about reading their evil counterparts' comic book on how Blunderpants and Nice Mr. Krupp can switch identities.
Underpants ties the alternate counterparts up, but when Good Harold states that nothing can go wrong, the worst thing is that something can go wrong. This causes a rain storm to turn Nice Mr. Krupp back into Captain Blunderpants and Captain Underpants is turned back into Nasty Mr. Krupp. Good George and Good Harold try snapping their fingers, but the snaps have no effect on either Nasty Mr. Krupp or Blunderpants as the rain is pouring too hard on them. Nasty Mr. Krupp can't figure out how he got outside, so he goes back to his soggy toilet paper-covered home to get back to bed not contributing to the plot of what is happening around him.
With Captain Underpants out of commission, Good George and Good Harold are forced to get away on Crackers while Captain Blunderpants flies after them with Evil George and Evil Harold in his hand. Good George soon finds the Shrinky-Pig 2000 in the treehouse which the gang can use to make the evil twin counterparts no longer a threat. Unfortunately it is too late as Blunderpants grabs the duo by their Shirts while the device falls into the hands of Evil George and Evil Harold. When all seemed lost at a point before Blunderpants can finish off Good George and Good Harold due to Captain Underpants not being in commission to save them, someone's voice stops him from doing so.
George and Harold's grandparents have arrived and order Blunderpants to put down their grandchildren, but Blunderpants just tightens his grip on the two children, so George and Harold's grandparents become Great Granny Girdle and Boxer Boy. They then place two curtains on them as capes before they defeat Captain Blunderpants and save their grandchildren. Good George notes he figured out what happened to the Super Power Juice that disappeared earlier. (Their grandparents drank it.)
However, Evil George and Evil Harold are prepared to use the Shrinky-Pig 2000 on their alternate twins, but Good Harold claims they can go right ahead as they are holding the device backwards. The evil twins believe this and turn the device around before activating it, resulting in themselves being shrunk. After giving the evil twins the punishment they deserve and George and Harold's grandparents fly off for a romantic dinner, the duo dehypnotize and shrink Hamsterzilla back into Sulu. They then take Evil George, Evil Harold, and Captain Blunderpants back into the Purple Potty and then they kick them back to their universe.
Good Harold states everything worked out perfectly, but this causes policemen to arrest them because they think they are the evil George and Harold, who robbed a bank while Captain Underpants was fighting Hamsterzilla. Good Harold again states that things cannot get any worse because they are going to jail for the rest of their lives. This time, however, Tippy Tinkletrousers (Professor Poopypants who changed his name at the end of Captain Underpants and the Perilous Plot of Professor Poopypants) comes and freezes the cops solid before he starts chasing Good George, Good Harold, Crackers and Sulu, and the book ends, once again, with Good George yelling "Oh no!" and Good Harold yelling "Here we go again!" 
Comics.
Comic 1: "The Preposterous Plight of Captain Blunderpants" (by evil Harold and George).
The alternative boys hypnotize Mr. Krupp into becoming Captain Blunderpants to steal things for them. One day, he is chased by an angry cop for stealing pizza and accidentally eats a mixture of foods which give him superpowers. He flies to the evil twins with their pizza. From that point, Nice Mr. Krupp turns into Captain Blunderpants by getting his head wet and de-hypnotized by the snap of a finger.
Comic 2: "Adventures of Boxer Boy and Great Granny Girdle" (by the real George and Harold).
One day, a UFO lands on Earth and opens a store that trades improved Robo-Grandparents for real grandparents, which everyone but George and Harold does. One day, their grandparents find the others inside a "slaves room" in the store, along with superpower candies, which they end up eating and then they become Boxer Boy and Great Granny Girdle, who chase the bad guys into the UFO. Boxer Boy turns the UFO into the shape of the candy, causing one Robot to bite the fuel line, and the explosion kills all the bad guys. The boys' Grandparents fly back and sets the kidnapped grandparents free.
Supporting characters.
The following characters appear on pages 46-47:
Reception.
Reviews for the book were mostly positive, with Kidsreads.com praising the entry.