When German agents infiltrate the telephone cable tunnels running under Southern California to tap sensitive information being transmitted over the telephone lines, George Martin, the general maintenance supervisor for the telephone company, moves to foil the spies. Martin sends for San Francisco engineer Lee Graham and his friend Miguel Gonzales to investigate the situation. After renting a room at Miller's boarding house, Graham reports to Martin. Unknown to Graham, his landlord is the covert leader of the spies. The spy ring operates from a fruit farm allegedly owned by Miller. After reporting to work, Graham hires Ann Carter, a discharged telephone operator, as his secretary. One day, Pete Dugan, a telephone lineman, accidentally discovers that the farm is a front for Miller, and is gassed to death by the spies. Planning to sabotage a load of aircraft motors, the spies capture Miguel, and when he refuses to cooperate, they kill him. Soon after, Pete's body is pulled from the ocean, and Graham notices that his hair has turned white, an effect of the nerve gas. Recalling that Miguel's dog's hair also turned white after the animal entered a room at the fruit farm, Graham begins to suspect Miller. To prevent the spies from tapping the wires, Graham installs a device to scramble all sensitive information. Frustrated, Miller and his gang kidnap Graham, intending to force him to decode the scrambled messages. Their plans are thwarted, however, when the telephone men come to the rescue and apprehend Miller and his gang.