Henry's mother and father are going to a Valentine's Day Sweetheart Dance, and Henry and Mudge are staying with Mrs. Hopper across the street. Henry's never been in Mrs. Hopper's scary, old stone house with its droopy trees and dark windows and gargoyle on the door. But inside, Henry discovers a houseful of surprises -- a grand piano, a room full of costumes, and eleven cats for Mudge to play with!