Henry and his dog Mudge are invited to a sleepover in Patrick's attic, where they watch monster movies, eat pizza, and enjoy a contest to determine whose dog is the best popcorn catcher.
Henry, planning a tasty Mother's Day surprise with the help of his father, must find a way to keep his big dog Mudge from devouring the treat before mom sees it.
When Henry and his dog Mudge go with Henry's parents to visit Great-Grandpa Bill in the home with lots of other grandpas, they lead them all on a wonderful adventure.