You are Shawn Maxwell, average 10-year-old boy. While out trick-or-treating one fateful Halloween night, Shawn takes a tumble into a deep, dark hole. Upon awakening, he finds himself in a twisted underground world, populated with terrifying monsters out to have Shawn for lunch.

But with the help of the friendly Muties, Shawn discovers that he now has the ability to shape-shift into powerful new forms. And he'll need all the help he can get on this harrowing journey back home...