Harold and George's time-traveling Purple Potty lands them in an alternate universe where everything seems delightful, but evil lurks just beneath the surface, and leads to a battle between Captain Underpants and his own evil twin.

When George and Harold hypnotize their principal into thinking that he is the superhero Captain Underpants, he leads them to the lair of the nefarious Dr. Diaper, where they must defeat his evil robot henchmen.

"Doggo likes routine: napping, eating, and more napping. Life is good. Life is good for Pupper, too, even though he worries a lot, about things like giant squirrels. If he were braver, he might even be a hero. Maybe even a hero who can fly! But heroes aren't afraid of giant squirrels ... When Doggo and Pupper meet a baby bird who also has worries, they are determined to help. Doggo and Pupper may not know how to fly, but they are very good helpers....

"OK. A lot of the stuff we said on the back of the last book was NOT TRUE. We now admit that The Bad Guys is WEIRD. We also admit that it features ALL the weird stuff we said it didn't. PLUS there's now a whole bunch of even weirder stuff involving an over-sharing bat, a really scary new character who lives in the woods, and a strangely confident little guy with a mullet. So, uh, don't relax, maybe FREAK OUT a little and have your MIND BLOWN by the...