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, postapocalyptic planet for all eternity. Buy 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!