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.
A snotty situation ensues when Melvin Sneedley, an academically-gifted student, turns into the Bionic Booger Boy and must be stopped by George, Harold, and Captain Underpants. By flipping the pages, the reader may view animated illustrations.
Captain Underpants and his friends, having dispatched Melvin Sneedley, the Bionic Booger Boy, must join the fray again when they realize they have forgotten about Sneedley's sidekicks, the Robo-Boogers.
Only Captain Underpants can stop the three evil space aliens who have invaded Jerome Horwitz Elementary School and turned everyone into lunchroom zombie nerds.