Captain Underpants and the Revolting Revenge of the Radioactive Robo-Boxers by Dav Pilkey (Book 10)

When we last saw our heroes, George and Harold, they had been turned into evil zombie nerds doomed to roam a devastated, post-apocalyptic planet for all eternity. But why, you might ask, didn't the amazing Captain Underpants save the boys from this frightening fate? Because Tippy Tinkletrousers and his time-traveling hijinks prevented George and Harold from creating Captain Underpants in the first place! Now, having changed the course of human history forever, they'll have to figure out a way to CHANGE IT BACK. Could this be the end for Captain Underpants?!!