Summary.  A “mad scientist”, the Professor, has rented the red and green mansion near the town Hawkins and the boys live in.  He has stolen from an old man the formula for a chemical mixture, which when activated creates a local region of invisibility that appears as a lavender light to those outside the affected region.  His bizarre plan to make money with this technology is to conceal an organ and a choir of boys on the stages of theaters, which he will call the Choir Invisible, and use them to awe audiences.  To this end, he has lured 13 boys to run away from home and live in the mansion, where he can train them and perfect his trick.  Shadow Loomis infiltrates the operation, and sends clues via radio to Hawkins.  In one of the more exciting sections of any Seckatary Hawkins book, nine entire chapters (chapters 28-36), encompassing pages 276-364, are taken up by the dramatic events of a single night when Hawkins and Jeckerson solve the mystery, and then they and many other law officers swoop down on the mansion and foil the plans of the mad professor.