In the not-so-pretty house where she lives with her dad and bunny Snowball, Annie plans to make some changes in the decor with the help of her next-door neighbor and cousin Henry and his big dog Mudge.
Annie and her pet rabbit, Snowball, invite Henry and Mudge to a dress-up birthday party, but there is some confusion about what the guests are supposed to wear.
While Henry's parents are at a Sweetheart Dance on Valentine's Day, he and his dog Mudge have a wonderful time staying at Mrs. Hopper's house, where they enjoy tea and music and dressing up in various costumes.
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.
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.