Joe Graham, an ace telephone line repairman in Los Angeles, turns down a promotion to become foreman offered to him by his supervisor, James Regan, and tells Regan that he prefers being a "trouble shooter" who uses his hands and wits to earn a living. When Joe's regular partner, Dan Sutter, refuses to work overtime, Joe agrees to work the night shift with Casey, a wise-cracking practical joker who has been transferred from Azusa. After dealing with a corpse during their first call, Joe and Casey then find Dan drunk in a gambling joint. Casey overhears the police discuss an upcoming raid on the casino, and Dan hears when Casey informs Joe about it. Dan tips off the owner, and the next day, Casey and Joe are blamed for the failed raid. Joe reveals Dan's involvement, and Dan is fired. That night, Joe makes up with his girl friend, operator Ethel Greenwood, with whom he had quarreled after discovering that she had dated Dan on one of the many occasions Joe had stood her up to work overtime. Their reunion is short-lived, however, for Dan arrives and tells Ethel that Joe lied to get him fired. Ethel and Joe quarrel again, and a few weeks later, Joe learns that Ethel quit the phone company to work in a real estate office run by Dan. The office is a front for racketeers George Martin and Max, who are tapping into the phone lines of the investment company next door to get tips on stocks. Martin gives Dan a check to give to Pearl Latour, Dan's sometime girl friend and partner, but Dan instead gives it to Ethel, who smudges the ink when she returns it to Dan. That afternoon, Joe and Casey investigate the investment company's complaint about a wiretap and discover the racketeers and Dan as they are tunneling to the company's vault. Casey bravely starts a fire to attract attention, and although he is wounded, the robbery is foiled. Dan gets away, and the police and Joe arrive at his apartment just as Ethel, who was to meet Dan there before embarking on a trip to Mexico with him, runs out of the apartment in hysterics. Dan has been shot, and although Joe maintains that it must have been suicide, the check with Ethel's fingerprints is in Dan's hand, and it leads the police to arrest her. When Ethel is indicted, Joe determines to find Pearl to question her. After an exhaustive search in Long Beach, Joe finds Pearl, who confesses that she killed Dan because he was trying to double-cross her. Before Joe can take Pearl in, however, a huge earthquake hits Long Beach, and Pearl is buried in debris. Joe and Casey rig an emergency phone line, and police Captain Flynn records Pearl's dying confession. Soon after, Ethel, now freed, finagles Joe into getting a marriage license when they are at city hall to witness the wedding of Casey and Ethel's roommate, Maizie.