It's back-to-school time, and Harry and Doug can't wait to start third grade. But when they get to school, so many things have changed that they feel like they're on Mars. At least two things remain the same: Harry is still gross, and Sidney is still his sworn enemy. So when Sidney kills Harry's pet spider, Harry wants revenge. And what better place to get it than on the class trip to a creepy old prison and copper mine. . . .