When Polly's dad and his new wife Veronica Kreep go on their honeymoon, Polly, Joy, and Petey Winkler are stuck alone with their weird new step-siblings, the Kreep kids, living in the Kreeps' spooky old mansion. Things get even worse when their sinister sitter, Ms. Pearl, shows up. Even sunny Joy and oblivious Petey have to admit that Polly might be right: something weird is going on in the Kreep mansion! The kids have to find a way to get rid of Ms. Pearl. And they just might need the Kreeps' help to do it.