Young Cam Jansen and the Knock, Knock Mystery

Cam, her best friend Eric, and Eric s mom are spending the day with Eric s grandparents. But Grandpa and Grandma seem too tired to visit. Every so often, a knock at the door wakes them up. But when they go to answer, no one is there. Is a Knock, Knock Ghost on the loose? It is up to Cam to solve the mystery!"