Plot summary

Millionaire sportsman Grayson Carter is found strangled to death at the Drexel Club in New York City, and becomes the fifth victim of the mysterious vigilante known as "Dr. Rx," a physician who kills crooks who have been acquitted of murder through various legal maneuvers. The next day, when famed private detective Jerry Church returns to the city after a year in South America, worried police captain Bill Hurd tells him that gangster boss Anthony Zarini has been charged with murder and is being defended by ace lawyer Dudley Crispin. When Hurd suggests that he investigate the case, Jerry tells the captain that he has quit detective work to return to his family's bond business in Boston. Later, John Crispin, Dudley's brother, attempts to hire Jerry for his sibling, and though the detective refuses the job, he agrees to see Dudley that night at his home on Long Island. Anxious to protect his client from retribution, Dudley offers Jerry $10,000 to take the case, with a $10,000 bonus if he finds Dr. Rx. The detective refuses once again, but reconsiders the offer after a female prowler is chased off the property. After leaving the Crispin estate, Jerry finds a note in his car, advising him to see Barney Scott in Syracuse. Immediately upon his acquittal, Zarini becomes the sixth victim of Dr. Rx. Jerry thinks that Ernie Paul, Zarini's partner, might have killed the gangster, knowing that the mad doctor would be blamed. Paul, however, has already left town, having been tipped off by "a woman's voice." Jerry then finds a microphone hidden in his apartment. With the help of his butler, Horatio Pitts Washington, Jerry follows the microphone's cable to the apartment below, which turns out to be the new home of his ex-girl friend, mystery writer Kit Logan. After sending Hurd off on a false lead, Jerry and Kit sneak off to get married. Kit then goes to Syracuse to see Barney Scott. Scott's mother warns Kit that her son, once a healthy policeman, was turned into a white-haired invalid while working on the Dr. Rx case. The next day, Jerry is abducted by Paul, who demands that he stay on the case in order to clear the gangster's name. Back at their apartment, Kit refuses to believe Jerry's kidnapping story and leaves him. When Jerry sends Horatio after her, the butler is abducted by Dr. Rx, who forces him to set up a meeting with his unsuspecting employer. Kit, Hurd and police sergeant Sweeney attempt to follow Jerry to the meeting, but the police sergeant rolls their car while making a fast turn. Back at his hideout, Dr. Rx makes plans to exchange Jerry's brain with that of an ape. At the Mayview hospital, meanwhile, Kit recovers from her automobile accident injuries, only to learn that Jerry and Horatio have been found. Like Scott, Jerry has become white-haired and is almost comatose. A doctor is brought in to examine Jerry, and when Dudley offers him his fountain pen, the disguised Jerry jumps from the bed and arrests the lawyer. Afterward, Jerry tells Hurd and Sweeney that Dudley, the real Dr. Rx, had used his fountain pen, which he filled with a poison that created the appearance of strangulation, to kill his victims. Jerry saw Dudley's pen while he was Dr. Rx's prisoner, and further explains that he had not escaped, but had been let go, as had Scott before him. Jerry and Kit make up, and all returns to normal, except Horatio's hair, which has truly turned white.