After traveling five years into the past, Tippy Tinkletrousers and his gigantic Robo-Pants show up right when Kipper Krupp and his gang ran away from the school. The four kids saw the Robo-Pants and were subsequently admitted into a mental institution. Mr. Krupp was blamed for this and was fired. Tippy then traveled to the future to find that Dr. Diaper, the Talking Toilets, and zombie nerds destroyed the world in a matter of weeks because Captain Underpants was not there to defeat them.
Tippy encounters Zombie Nerd Harold. He lifts his foot above Tippy Tinkletrousers' head and stomps him into a red stain. Zombie Nerd George just laughs his head off.
It turns out that Zombie Nerds move slowly enough that Tippy Tinkletrousers was never in any real danger. When Zombie Nerd Harold lifted his foot above Tippy's head, he screamed in horror several times until his throat got sore, so he went to the store and bought some cherry throat lozenges. While he was there, he purchased a Giant-Novelty-Ketchup-Pack, placed it under Zombie Nerd Harold's Gigantic Foot & Super-Sized Shoe, and walked away. This is what produced the red stain in book 9.