The Mad Scientists' Club tackles a quiet mystery when their homemade seismographs begin recording strange, irregular tremors every night between midnight and four a.m. While the boys initially see the project as tedious work—hiking to distant sites to replace graph paper—Henry Mulligan and Jeff Crocker uncover an intriguing pattern in the data: small, random vibrations occurring consistently at the same hours, defying mechanical explanation. As the club debates possible causes, from drunken stumbles to secret machinery, they realize the tremors must originate from a specific location in Mammoth Falls, leading them to deploy a covert nighttime investigation. Armed with radio communication and a radiosonde transmitter, they navigate shadowed downtown alleys, listening for the source of the thudding sounds, only to find themselves inches from a basement window where the mystery unfolds. The adventure blends meticulous scientific observation with the thrill of a real-time detective hunt.