A disreputable doctor, L. E. Thai, acquires a mysterious gas that can temporarily paralyze people. An ex-convict, Gruesome, sees the potential for criminal use and enlists Thai's help to rob a bank. However, Tess Truehart witnesses the robbery and alerts detective Dick Tracy, who begins to investigate. After their getaway car is destroyed, Gruesome escapes, but his accomplice is captured and taken to the hospital. When Dr. Thai decides to turn himself in, Gruesome kills him. Tracy sets a trap by pretending to be the captured accomplice in the hospital and lures Gruesome to his hideout. Tracy's plan almost fails when his partner loses the trail, but Gruesome is eventually apprehended by the detective.