Detective Jerry Carter, a special agent for the International Jewelers' Association, is assigned the task of protecting the $250,000 Jarvis diamond, known as "The Devil Diamond," because some believe the jewel is cursed. Lannings, a diamond-cutting expert, is hired to the cut the raw Jarvis diamond into multiple finished stones. Arthur Stevens, director of Van Groote and Company Jewelers and part-owner of the Jarvis diamond, hires a group of gangsters, lead by "Professor" Morlan, to steal the diamond once it is cut, and thus, untraceable. Stevens' plan is to sell the stolen stones while also collecting the insurance money. When Lannings and his daughter Dorothy take a room at a small inn to cut the stone, Morlan and his gang, pretending to be boxing trainers, move into the inn as well. The gangsters have, as their dupe, Lee, a one-time messenger who thinks they are preparing him for a boxing career. Jerry also takes a room at the inn, pretending to be a writer seeking inspiration. Although Jerry must watch the Jarvis diamond, he finds himself distracted by his attraction to Dorothy. Meanwhile, Lee receives little attention from his trainers, but attracts the interest of Yvonne, a teenager staying at the inn with her aunt. Jerry immediately suspects Morlan and his gang, and he questions Lee about their training methods. That night, Jerry, Dorothy, Lee and Yvonne go to a neighboring roadhouse, where Jerry spies upon Morlan and his gang. Morlan recognizes Jerry's suspicions, and has a bar fight started, hoping to get Jerry out of his way. The next day, the Jarvis diamond finally arrives and Lannings begins cutting the stone. Lee catches Jerry looking in Lannings' window, so Jerry tells the young man his true identity and acquires Lee's assistance in his investigation. Morlan meets with Stevens and asks for an increase in his portion of the proceeds. When Stevens refuses to increase his cut, Morlan has him killed during the drive home. When Dorothy reads the headlines proclaiming Stevens' death, thus proving the diamond's curse, she worries aloud to her father, who pays her little attention. Then, after he finishes cutting the diamond, he is kidnapped by an unknown assailant. The cut diamonds, however, are hidden in the teapot, and after Dorothy hysterically tells everyone about her father's disappearance, Yvonne's aunt finds the hidden diamonds. Unable to find Dorothy, the aunt re-hides the diamonds in her newspaper, which is taken by Chuck, one of Morlan's hoods, who wants it to "read the funnies." Yvonne sends Lee after the paper, and when he grabs it, the diamonds spill out. Lee is knocked unconscious, and the gang prepares to make their getaway. Jerry and Yvonne find the unconscious Lee, then discover Lannings in the basement. Jerry and Lee go after the gang, and after the gangsters' car goes out of control, then stalls after a high-speed chase, Jerry arrests Morlan and his cohorts. With the gangsters behind bars and the diamonds finally safe, Lee wonders if Jerry has deserted Dorothy, as he is now a "hero," but Jerry returns and takes Dorothy in his arms. As Lee watches the couple embrace, Yvonne sneaks a kiss and tells him: "You're so cute!"