George and Harold love Christmas, but they've decided that the old traditions could use an upgrade. Their solution? Blissmas - a cooler, more modern version of Christmas, including a Mechalition Derby, holi-DJs instead of Christmas carols, lasers instead of twinkle lights, and a totally jacked Santa Claus. But as with all of George and Harold's genius ideas, Blissmas has some unexpected consequences. If they don't act fast, Christmas could be gone forever! Can Captain Underpants defeat Mechanaclaus and save the holidays?