It's almost Halloween and Tom and Lucy Little are going to scare the adults! But before they can come up with a plan, their Uncle Nick retires from the army and comes to live with the rest of the Little clan. The Littles don't know about Nick's pet Mus Mus, a White House mouse, and they are caught off guard and scared silly when the mouse appears in their home. But before Halloween is over, Tom and Lucy will have the last laugh as they play the best prank ever on their family.