Blonde good-time girl Vivian Heldon (Jan Sterling), who lives in a cheap rooming house in a working-class section of Boston, run by the inquisitive and neurotic Mrs. Smerrling (Elsa Lanchester), goes out one night after a phone conversation with her boyfriend, proclaiming that she's got big plans and might even move to a nicer place. After putting in her shift as a waitress at a cheap dive called The Grass Skirt, she latches onto Henry Shanway (Marshall Thompson), an innocently drunk patron, who's trying to wash away his sadness over his wife's stillborn child. She uses Henry's car with him in tow to drive out to Cape Cod, then strands him on foot and meets her boyfriend -- but when she arrives, he puts a bullet into her, then strips the body, throws it into the sea, and drops the clothes and the car into a lake. Six months later, an ornithologist from the cape spots the skeleton of a human foot sticking up through the sand.
Enter Lt. Peter Morales (Ricardo Montalban) of the Boston PD; he and his partner on this case, Det. Sharkey (Wally Maher), bring the bones to Dr. McAdoo (Bruce Bennett), of Harvard University's forensic medical laboratory. Over the next few days, McAdoo and his staff are able to determine the gender, age, and general appearance of the person to whom the bones belonged, and that this is a case of murder -- and that the victim was pregnant. Morales and Sharkey, combing through what they know about the victim and the missing persons records of six nearby states, eventually tie the skeleton up with Vivian Heldon, who disappeared on just about the same day the victim was killed, and also to Shanway's car, which he reported stolen that day. The poor slob, who is merely trying to cover up a drunken lapse from his wife (Sally Forrest), acts guilty enough and lies about just enough so that Morales is certain that he's the murderer. His investigation isn't helped by the interference of Mrs. Smerrling, who sold Vivian's belongings when she didn't return to her room, and now seems fixated, even obsessed with the details of the case and its connection to her rooming house. While the police tighten the screws on Shanway, she backtracks Vivian's phone calls and makes contact with the woman's boyfriend, James Joshua Harkley (Edmon Ryan), member of a wealthy Boston family, and a married man; she also manages to steal a vital piece of evidence. But instead of turning it over to the police, she uses it to blackmail Harkley.
Meanwhile, the district attorney sets an early trial date for Shanway, but with the opening arguments only a week away, Morales begins to develop doubts about Shanway's guilt, in addition to harboring his own sympathy for Grace Shanway, whose life is being gradually destroyed by the prosecution on her husband -- not that Morales thinks he's innocent, but there's enough that's not right about the case, including the missing murder weapon, that he's not 100-percent sure. And that's when Vivian's friend and neighbor, Jackie Elcott (Betsy Blair) reports how strangely Mrs. Smerrling is acting, and the fact that she's got a gun. But before they can question her, Harkley kills Mrs. Smerrling -- now it's a race between Morales and Harkley to see who can get to the murder weapon first.