In New York City's Central Park, a woman is found shot to death in a stolen car. Police captain Lundquist assigns the case to veteran homicide lieutenant Corrigan and Detective Frank Tobin, a college-educated "rookie" from the forensics bureau. Tobin immediately impresses the skeptical Corrigan when he examines the stolen car and deduces that the unidentified victim was not shot in the car and that her killer is a man of average height. Later, as the two detectives are about to enter the autopsy room at the police morgue, a crazed old man is caught cutting the victim's body with a knife. Tobin chases the old man into the morgue's basement and shoots him before he can attack Corrigan with the knife. Corrigan recognizes the old man as "Billy Alcohol," a longtime drunk, and speculates that he was paid by the woman's killer to mutilate her body. The medical examiner then reveals that although Billy removed a tattoo from the victim's arm, a photograph of the tattoo had already been taken. In addition, the medical examiner notes that the corpse had fallen arches and purple ink on her thumb, suggesting that she may have been a waitress. Forensics chief Captain Gavin then announces that the fatal bullet was filled with sand to prevent ballistical identification. He also presents Tobin and Corrigan with an unusual grass sample found on the stolen car's gas pedal and advises them to talk with a botanist about its origins. While Corrigan leaves to conduct inquiries at restaurants around the city, Tobin discusses the case with attractive botanist Mary Mahan. Mary quickly identifies the grass sample as a species found only in the Midwest, but then uncovers a report that refers to an isolated sighting of it in the Bronx. Before Tobin can follow up on Mary's findings, he is ordered to accompany Corrigan, who was unsuccessful in his waitress search, to some tattoo parlors. One tattoo artist identifies the victim's tattoo as the work of Brooklyn parlor owner Johnny Marseille. At Johnny's, the detectives learn that the woman came in twice to his parlor, first to get a joint tattoo with her husband, Merchant Marine Al Raditz, whose ship was sunk during the war, and a year later, to add a Marine emblem to it. After Johnny remembers that the woman worked in a Brooklyn café, Tobin and Corrigan canvas all the local "beaneries" and finally corner her boss, Joe Canko, who reluctantly informs them that her name was Lottie Morrell. Through information supplied by Joe, Tobin and Corrigan find Lottie's last known address in the Bronx and discover that she had a number of GI insurance policies under various names. As they are searching her apartment, the detectives realize they are being watched by someone, but are unable to catch the man. Later, after Lottie is identified as a convicted swindler and bigamist, Tobin joins Mary in a search for the Midwestern grass, but is called away to investigate Johnny's beating death. From the murder scene, the police secure fingerprints that identify Johnny's killer as Raditz, Lottie's first husband, who, they learn, fled to Canada during the war. The detectives speculate that Raditz and Lottie were blackmailing each other, and that Raditz killed Lottie. Gavin then reports that a microscopic examination of sand taken from Raditz's bullet suggests that he was working as a granite cutter at the time of Lottie's murder. Putting the grass and sand clues together, Tobin has a hunch that Raditz might have been working on a tombstone at a Bronx cemetery the day he killed Lottie. Tobin's guess proves accurate as he discovers a recently dug gravesite where a patch of the Midwestern grass is growing. Tracing the tombstone to Fisher Monumental, Tobin and Mary then question the owner, who, unknown to them, is being threatened into silence by a hidden Raditz. Eventually, however, Tobin deduces Raditz's presence and, after sending Mary to get backup, becomes involved in a shootout with the murderer. Although Tobin is wounded in the exchange, he kills Raditz just as Corrigan and the Bronx police arrive with Mary. After Corrigan chastises his partner for taking Raditz on singlehandedly, he blesses Tobin's budding romance with Mary.