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? Tippy Tinkletrousers's time-traveling hijinks prevented George and Harold from creating Captain Underpants in the first place. Could this be the end for Captain Underpants?!