When Billy discovers his shadow, he queries Grim on where shadows come from. Grim responds by taking him and Mandy to the Shadow World, where their shadows are the respective characters' polar opposites (Billy's shadow is even stupider than Billy, and Mandy's is very nice). While Mandy tries to get her shadow to toughen up and be exactly like her, Billy and his shadow accidentally open a door that releases millions of Billy clones from other universes into the Shadow World.
—Anonymous