Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather, and they are about to encounter a mystery at midnight!   The Alden Children are competing in an invention convention held at the home of a famous inventor known for her one-of-a-kind clocks. The clocks sound loudly at every hour, especially at midnight, when the Aldens are awakened by a concert of chirps, chimes, clangs, and gongs. But the clocks aren't the only noise in the house at midnight. Someone or something is making a mysterious tapping sound. Is the late-night tapper inventing trouble for the convention? The Aldens are determined to find out!