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.
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.
After noticing that a visiting hummingbird likes her pink petunias, Annie and her rabbit Snowball, accompanied by Henry and his dog Mudge, fill the garden with pink objects to attract more hummingbirds.
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.