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Warren Quimby (Richard Basehart) has a problem that he won't admit to: his wife, Claire (Audrey Totter), wants more fun and excitement out of life than the quiet, soft-spoken pharmacist can give her. So she steps out on him regularly, something that Warren refuses to recognize until the day she packs her bags and announces she's moving in with Barney Deager (Lloyd Gough), the flashy, high-living salesman whom she's been seeing on the side. Warren goes to Deager's, to ask her to come back, and gets beaten up by his rival. The worm suddenly turns in his own quiet way of doing things, and Warren decides he's going to get even -- he's going to murder Deager, but to do it and not get caught, he bides his time. He begins building a new identity for himself, changing his appearance (starting with getting rid of his thick glasses in favor of then-newly-invented contact lenses), and establishing his alter ego of "Paul Sothern" at a new address. He then starts making threatening calls to Deager, identifying himself as Sothern, in order to establish a history of enmity between the two. It's all going according to plan -- he will kill Deager and the blame will fall on "Sothern," who, of course, doesn't exist, and will have vanished. But then he meets Mary Chanler (Cyd Charisse), a neighbor at "Sothern"'s apartment building, and the two become attracted to each other. For every step forward that he takes in his plan for murder, he also finds himself with new possibilities in his life, growing out of his relationship with Mary. Finally, the night comes when he breaks into Deager's home and is about to kill him, when it dawns on Warren that he doesn't need to do this....He awakens Deager and tells him what almost happened and that he's leaving, and Deager is welcome to Claire, and leaves. It seems as though he's averted disaster when Claire shows up later that night, pleading for reconciliation and finally, when pressed, telling him that someone has murdered Deager. The man has been shot, and the police, led by lieutenants Bonnabel (Barry Sullivan) and Gonsales (William Conrad), are hunting for Paul Sothern. Now Warren is scared for his life, having done his best to frame himself, and Claire isn't helping by trying to run interference -- he can't tell if she's trying to protect the two of them, implicate Sothern, or protect herself, and he's too scared to say almost anything at all to the detectives.