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. Tippy Tinkletrousers and this 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.