Captain Underpants and the Revolting Revenge of the Radioactive Robo-Boxers

When we last saw George and Harold, they had been turned into evil zombie nerds. But why didn`t Captain Underpants save them from this fate? Because Tippy Tinkletrousers and his time-travelling hijinks prevented George and Harold from creating Captain Underpants in the first place! Could this be the end for Captain Underpants?!!