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 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?