The last time we left our heroes, Melvin Sneedley had been freed from his booger/cyborg enhancements and returned to normal. Unfortunately, the pieces that made up his enhancements are now alive themselves, and they won't rest until Captain Underpants is destroyed.
The book title is pretty misleading. Seeing as how the Bionic Booger Boy himself has been dealt with, the first part of the title really doesn't make much sense. Unfortunately, neither does the subtitle. The Robo-Boogers, surprisingly, play a very small role in the book. Most of the story centers around Geoge and Harold trying to figure out how to return Melvin and Mr. Krupp into their own bodies with the Robo Boogers only showing up at the beginning and end of the stories. Still, it's always nice to see continuity in a story, and this book not only continues where the previous left off, but sets up the next one. And what a story it looks to be!