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. 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. 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.
The duo and Crackers go to Mean 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 Mean Mr. Krupp answers, Good George and Good Harold snap their fingers to turn Mean 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 Mean Mr. Krupp. Good George and Good Harold try snapping their fingers, but the snaps have no effect on either Mean Mr. Krupp or Blunderpants as the rain is pouring too hard. Mean 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.
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!"
- The gender of Crackers is brought into question, when he/she/it saves Good George and Good Harold despite getting hypnotized to be bad by the 3-D Hypno Ring (according to Book Five, the effects of the 3-D Hypno Ring are reversed on any women, such as Ms. Ribble/Wedgie Woman) and when all the "He's" were italicized in Good George and Good Harold's quotes. This is further confused by the fact that the narrator only refers to Crackers by name and species (Crackers, plucky pterodactyl etc). The tenth book later revealed that Crackers is indeed female, so this means that the 3-D Hypno Ring in Captain Blunderpants' universe works oppositely on women, just like the regular 3-D Hypno Ring.
- It was originally stated that Boxer Boy and Great Granny Girdle were going to appear in the ninth book, however Dav Pilkey changed his mind most likely since they appeared in this book.
- The subtitle of this book, back when it and Captain Underpants and the Terrifying Re-Turn of Tippy Tinkletrousers were to be both parts of one story, was originally "The Clash of the Colossal Kung-Fu Monkeys from Beyond Infinity". However, no such characters appear in this book.
- The book that the librarian offers the boys shortly after they arrive, Mommy Has Two Heathers, is a parody of the 1989 book, Heather Has Two Mommies.
- Actually, considering that this takes place during (their) Banned Books Week, and Heather Has Two Mommies is banned in our universe, technically "Mommy Has Two Heathers" shouldn't be banned.
- At the 75th page of the book, the illustration depicts a sign saying: "'Schools is educationy' ; A message from our president." This might be an indirect way of criticising or parodying American president George W. Bush and his frequent grammar mistakes known by some as Bushisms.
- In the inside comic book about Captain Blunderpants, when Blunderpants steals pizza, it is called "Grand Theft Pizza" which is a spoof of the crime "Grand Theft Auto".
- If Melvin Sneedly does not have any intelligence in Captain Blunderpants' universe, then there should not be alternate versions of the Turbo Toilet 2000 and the Robo-Boogers, since they were created by his inventions.
- The title of chapter 19, "Whenhamstersattack.com", is a real website, first created to promote Bionic Booger Boy (Book 6). It might be possible that it was named after the website because Sulu is a hamster, and he is attacking and the attack is mentioned in the site.
- On page 72, a sign in the library shelf says, "Reading Might Offend You... Why Take a Chance?" This was first seen in the preceding book.
- In the double-page 46-47 illustration, Dr Diaper (Book 1) is a policeman, The Turbo Toilet 2000 (Book 2) a crossing guard, Zorx, Klax & Jennifer (Book 3) and Professor Poopypants/Tippy Tinkletrousers (Book 4) are firemen, and Trixie, Frankenbooger, & Carl (Books 6-7) are road-workers and a postman respectively.
- The only villains not seen are the Wicked Wedgie Woman and the Bionic Booger Boy. This might be because George and Harold haven't hypnotized Ms. Ribble in Captain Blunderpants' universe and Melvin has already been separated from the Robo-Boogers.
- Captain Blunderpants is a parody of Ultraman.
- The cover of George and Harold's second comic has a subtitle of "An Epic Novella by George Beard and Harold Hutchins", mirroring the Captain Underpants subtitles of "The ___th/nd/st/rd Epic Novel by Dav Pilkey".
- This is the first book where the intro comic is held by somebody rather than the comic being somewhat floating in mid air.
- When Evil George and Evil Harold find the Goosy Grow 4000, it is shown to fit in the evil George's palm, but in the 4th book when Good George and Good Harold returned to normal size it was smaller than a penny.
- Ironically, the Good George and Harold consider their counterparts' work terrible compared to their own.
- This is the first time there are two To Make a Long Story Short chapters in a book. Usually there is only one per book.
Differences between the two universes
|Underpants' Universe||Blunderpants' Universe|
|There is only one book in the library, and the librarian, Ms. Singerbrains, thought she was fired (since she did not understand what Melvin, who switched bodies with Mr. Krupp, said in book 7).||The library is full of books and the librarian is Ms. Singerbrains.|
|There is no school nurse in the school. There are only a box of band-aids and a rusty saw in case someone is sick or hurt.||There is a school nurse in the school (A male one to be precise). The nurse's office is right next to the cafeteria.|
|All the teachers except Ms. Ribble and Mr. Fyde (Mr. Fyde is retired) are mean.||All the teachers (possibly except Ms. Ribble and Mr. Fyde) are nice.|
|Mr. Krupp thinks that George and Harold's jokes are rude and offensive.||Mr. Krupp thinks that George and Harold's jokes are funny.|
|Melvin is clever, smart, and intelligent.||Melvin is dumb, has no intelligence and is stupid.|
|The school emphasizes knowledge and penalizes imagination.||The school emphasizes imagination and penalizes knowledge.|
|Harold wears a T-shirt and George wears a collared shirt and tie.||George wears a T-shirt and Harold wears a collared shirt and tie.|
|George and Harold are mischievous but good-natured.||George and Harold are evil with a taste for destructive chaos.|
|The cafeteria smells dirty, greasy, and moldy (i.e. cafeteria food).||The cafeteria smells good.|
|Mr. Meaner is mean, fat, and coaches the Purple Dragon Sing-A-Long Friends.||Mr. Meaner is nice, fit, and coaches the Knuckleheads.|
|George and Harold's past enemies are evil.||George and Harold's past enemies are good.|
|While working on the comics, George does the writing and Harold does the illustrating.||While working on the comics, George does the illustrating and Harold does the writing (which is possibly why the drawings are more accurate and the words are spelled right and are written better, unlike the regular George and Harold.).|
|George's writing is everywhere and sometimes crossed out and misspelled (to be fair, though, he is only 9 years old).||Harold's writing is aligned and spelled correctly.|
|Harold's artwork is slightly sketchy.||George's artwork is perfect and accurate.|
|The underwear hero created by George and Harold is Captain Underpants.||The underwear villain created by George and Harold is Captain Blunderpants.|
|Mr. Krupp turns into Captain Underpants by the snap of a finger.||Mr. Krupp turns into Captain Blunderpants by pouring water on his head.|
|Captain Underpants turns back into Mr. Krupp by pouring water on his head.||Captain Blunderpants turns back into Mr. Krupp by the snap of a finger.|
|Captain Underpants' war cry is "Tra-La-Laaaaa!".||Captain Blunderpants' war cry is "La-La-Traaaaa!".|
|Mr. Krupp is mean and wears a toupee.||Mr. Krupp is nice and is bald.|
|Captain Underpants is nice and is bald.||Captain Blunderpants is mean and wears a toupee.|
|At the end of the nice George and Harold's comic, they show a sketchy picture of a treehouse with the words saying: "Tree House Comix Inc." written below it.||At the end of the evil George and Harold's comic, they show a far more accurate picture of a treehouse, with the words saying: "Evil Tree House Comics, LLC." written below it.|
|George and Harold own pets.||George and Harold do not own pets.|