Captain Underpants and the Preposterous Plight of the Purple Potty Peopleis the eighth epic novel by Dav Pilkey.
The book first starts off with a rant about how grown-ups (such as your parents) encourage something initially
when you are very young and discourage it greatly as you grow older. Then it goes to the staff of Jerome Horwitz Elementary School who are anything but trustworthy and are extremely cruel to children. However George and Harold manage to outsmart them frequently since they aren't very bright.
Getting back to the main story, 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 humor (Like Captain Underpants does). After they realizes 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 super villain named Captain Blunderpants. Sulu and Crackers are kidnapped by Evil George and Evil Harold. They are hypnotized to be evil. 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 raise havoc into the town of Piqua. Good George and Good Harold take the 3D Hypno Ring so that they could dehypnotize Sulu and 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. Good George and Good Harold fly over on Crackers' back, and decide that they have to drink the super power juice, but unfortunately, 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 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 snap their fingers to turn Nasty Mr. Krupp into Captain Underpants. The latter then defeats the "Giant-Evil Sulu". The evil trio notice this after robbing a bank, so Underpants now must fight his evil counterpart. However before the 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. 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. Before Blunderpants can finish off Good George and Good Harold, 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.
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 Sulu to normal. 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 Sulu. 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 and Good Harold. The book ends, once again, with Good George yelling "Oh no!" and Good Harold yelling "Here we go again!"