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.
While trying out the spy glasses, spy hat, and other items in their new spy kit, seven-year-old Henry and his dog Mudge make a new friend and form a spy club.
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.
At first Henry's very neat cousin doesn't like the cookies from under his bed or Mudge's slobbery kisses, but when they all play frisbee, she begins to enjoy her visit.
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.