Thirteen-year-old Lacey Berge is furious. Her dad's dragged her to her late great-uncle's mansion in the mountains of Vermont, and taken her stepsister Cal along for the ride. Lacey can't stand that little know-it-all, and nothing her dad can say or do will force them to bond.

But soon after they arrive, an emergency forces Lacey's dad to leave the girls on their own. When they take the opportunity to explore, they discover a secret lab and accidentally open a portal to another world, unleashing a horde of mischievous monsters into the mansion!

Can these reluctant siblings put aside their differences long enough to get rid of the creatures before Lacey's dad gets back?