Cam and her best friend Eric are on their way to visit Eric"s grandparents. When they arrive, they are greeted with plates full of cookies! But Grandma Minnie and Grandpa Max don"t seem as excited about the cookies--in fact, they look tired. Grandma Minnie explains that someone keeps knocking on the door and waking them up. But when they go to answer, there is no one there. Cam must use her photographic memory to solve this knock-knock mystery!