After escaped convict Gruesome seeks out his former partner in crime, Melody, a piano player at Hangman's Knot bar, he goes with Melody to see the "Doctor," a crime boss, about participating in a "big job." The Doctor works out of a plastics factory, and while Gruesome is waiting in a laboratory to speak with him, he opens a vial containing an asphyxiating gas. After inhaling the gas, the enormous convict stumbles out of the factory and makes his way back to Hangman's Knot. There he succumbs to the gas's effects and is picked up by policeman Pat Patton, who, believing that Gruesome is drunk, drives him to the police hospital. A doctor, however, declares Gruesome dead and sends him to the morgue. Suspecting foul play, Pat reports his findings to his boss, detective Dick Tracy, who is talking with scientist Dr. A. Tomic about recent threats on Tomic's life. A few minutes later, Gruesome wakes up in the morgue, knocks out Pat, steals his gun and escapes. Gruesome then returns to the plastics factory and informs the Doctor's assistant, X-Ray, that he is now the Doctor's partner. Shortly after, the Doctor's gas is activated in a bank, causing everyone there, except Tracy's girl friend, Tess Trueheart, to freeze in mid-step. Gruesome and Melody walk in and steal $100,000 from their paralyzed victims, but Tess, who was in a telephone booth when the gas went off, is able to call Tracy. Before Tracy arrives, however, Melody shoots and kills a policeman and escapes with Gruesome and X-Ray. The robbery attracts the attention of crime reporter Dan Sterne, but Tracy, concerned that news of the Doctor's gas will cause costly bank runs, convinces him to suppress the story until two o'clock that morning. Tracy then goes to Dr. Tomic's office, where he learns from Tomic's assistant, Irma M. Learned, that the doctor has vanished. Tracy questions a nervous Irma about the experimental chemicals Tomic was working on and insists on taking a sample from one of his bottles. While Tracy is having the chemical analyzed, Pat finds Melody and Gruesome at Hangman's Knot, then pursues them as they flee and causes them to crash their car. Melody is seriously injured, but Gruesome eludes capture. Later, Tracy brings all of the robbery witnesses to view the hospitalized Melody, and one man, Dr. Lee E. Thal, makes a positive identification. Unknown to Tracy, however, Thal is actually the Doctor, who with Irma's help, stole Tomic's gas and planted it in the bank. After Irma meets secretly with Thal, her lover, she is visited by Tracy, who has since learned that the sample he took was only water. Although Irma denies any wrongdoing, Tracy suspects her and has Pat follow her as she rushes off to meet Thal. Before Pat can intercept her, Gruesome drives up and shoots her. Tracy then learns that Melody has died, but plants a radio report that he is alive and is going to talk. As hoped, Gruesome, who has since murdered the double-crossing Thal at his factory and incinerated his body just as he had incinerated Tomic, believes the story and sneaks into the hospital with X-Ray. Gruesome kidnaps the heavily bandaged Tracy from Melody's bed and is about to incinerate him at the factory when Tracy jumps him. After a fierce fight, Tracy finds Thal's gun and shoots Gruesome, meeting his two o'clock deadline with only minutes to spare.