Captain Underpants and The Big, Bad Battle of the Bionic Booger Boy: Revenge of the Ridiculous Robo-Boogers:

Clearly one epic novel could never contain the hugely disgusting tale of the Bionic Booger Boy; it makes perfect sense that his story would spill out into Dav Pilkey's seventh epic novel in the Captain Underpants series, Captain Underpants and the Big, Bad Battle of the Bionic Booger Boy Part 2: The Revenge of the Ridiculous Robo-Boogers. It all started in the sixth epic novel when Melvin Sneedly, a nerd, invented a "combine-o-thingy" that morphed Melvin, a robot, and (accidentally) mucus from his sneeze and created the monstrous Bionic Booger Boy. Unfortunately, in the chaos that ensues, the school principal Mr. Krupp's brain switches to Melvin's body and vice versa (making for a very unhappy school secretary). The diabolical Melvin becomes power-hungry when he realizes that he might be able to keep Mr. Krupp's Captain Underpants superpowers for himself! Can George and Harold stop Melvin from tyrannizing the school--and the world--in his new guise as principal and superhero?