The Deadly Whisper

Professor Soames has developed a sonic rifle, capable of projecting destructive waves of ultrasonic vibration, accurate to within a centimetre over thirty feet. But three unscrupulous saboteurs intend to use Soames' rifle to bring down an experimental aircraft, holding the Professor's daughter hostage to ensure his complicity. When Father Unwin becomes suspicious and investigates, he too is held prisoner in Soames' house.