Dave Mallory (Boris Karloff) has his invention stolen from him by his former business partner, Ranger (Samuel Hinds), who owns a burglar alarm system company. Despite his failing health, Mallory creates a new and superior alarm system with the hope of providing security for his daughter, Joan (Jean Rogers). He sells it to Ranger's company on a royalty basis, but soon discovers that Ranger has no intention of using the new invention. Enraged, Mallory attempts to force Ranger to uphold their original agreement by secretly breaking into stores using the system and rendering it ineffective with his own specialized apparatus. As his business is at risk, Ranger assigns his trouble-shooter, Travers (Warren Hull), to track down Mallory.
A group of thieves kidnaps Mallory in order to use his invention to commit robberies. They also abduct Mallory's daughter, forcing him to cooperate with them. The gang proceeds to terrorize the city by burglarizing every commercial establishment that uses Ranger's alarm system. With the help of Travers, Mallory escapes the gang's hideout and convinces Ranger to follow his plan for capturing the criminals. After Joan is rescued and the gang is rounded up by the police, Ranger promises to fully compensate Mallory for his invention.