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. But 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.