Captain Underpants and the revolting revenge of the radioactive robo-boxers

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-travelling hijinks prevented George and Harold from creating Captain Underpants in the firs