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Invisible Killer was the second effort from Producers Distributing Corporation, the enterprising poverty-row movie firm that would eventually matriculate into PRC Pictures. Based on Carter Wayne's novel Murder for Millions, the story concerns a trio of mysterious murders, each committed in the same bizarre fashion. Girl reporter Sue (Grace Bradley) and her police-officer boyfriend Jerry (Roland Drew) suspect that all three killings were perpetrated by the same murderer (duh!), and set about to prove it. The gimmick is a tiny poison pellet concealed in a telephone receiver, which is released into the air when the victim-to-be answers the phone. But knowing how it's done isn't quite the same thing as knowing who, and that's what Sue and Jerry want to find out.