Gretchen and her stepbrother Clark are absolutely dreading their vacation. They have to stay at their grandparents' house, which sits right in the middle of a dark, muddy, and isolated swamp. Grandpa Eddie cannot hear a thing, and Grandma Rose is completely obsessed with baking pies. It is shaping up to be the most boring trip of their lives. But boredom quickly turns into terror. There is something deeply strange about the old swamp house. The biggest mystery is the locked room upstairs. Their grandparents refuse to talk about it, but Gretchen and Clark can hear heavy, thumping footsteps and low, angry growls coming from behind the heavy wooden door. When the kids finally discover what is hiding inside, they realize they are trapped in the house with a terrifying creature. Now, they must figure out how to defeat a monster before it escapes!