The Riddle of the Lonely House

"If you hear me tap three times," Alan warns, "let me in as quick as you can!" Then Alan disappears, leaving Pete and his sister Chris alone in the shivery darkness of the deserted house. What is the secret of the lonely old mansion? Why has the missing document never been found? What does it all mean, the strange footprint, the broken door, the sound of footsteps in the empty house?