A police detective tries to convict a mob boss by going to the man's girlfriend.

One night, Fante and Mingo, minions of organized crime boss Mr. Brown, apprehend Susan Lowell, Brown's girl friend, after she runs away from them. Upon catching Susan, they take her to a restaurant, where she collapses from an overdose of sleeping pills. Unknown to the criminals, they are followed by policeman Sam Hill, who retrieves Susan's purse and takes it to police lieutenant Leonard Diamond. Diamond, who has just been chastised by Capt. Jeff Peterson for spending too much money on his relentless and unsuccessful pursuit of Brown, is in love with Susan, and immediately rushes to the hospital to question her. In order to keep Susan from being released to Brown, Diamond arrests her for attempting suicide, but when he questions the fatalistic young woman, all she tells him is that recently Brown was brooding in his apartment and writing the name "Alicia" on the foggy windows. Diamond's interrogation is interrupted when Brown's lawyer arrives with a writ to release Susan, and so Diamond decides to arrest all of Brown's men and question them about Alicia. The underlings are baffled by their arrests, and only one, Brown's right-hand man and former boss, Joe McClure, knows the meaning of the name. Brown agrees to take a lie detector test, but grows angry when Diamond mentions Alicia and the name Bettini, and storms out. That night, Peterson again reprimands Diamond for wasting police resources, and the depressed Diamond spends the evening with his sometime girl friend, burlesque dancer Rita. The next night, Rita tells Diamond she has heard that Brown has put out a contract on him, but Diamond shrugs off her concerns. As he is leaving the theater, Diamond is kidnapped by Mingo, Fante and McClure, who take him to the hidden cellar undernearth Brown's hotel. There, when Diamond refuses to answer Brown's questions, the gangster tortures him by inserting McClure's hearing aid into his ear and holding the amplifier next to a blaring radio. Diamond passes out from the pain, and in order to make him look drunk, Brown forces him to drink an alcohol-laden bottle of hair tonic. Mingo and Fante then leave the staggering Diamond at Peterson's apartment, where Diamond recovers and tells Peterson the story. Regretting his dismissal of Diamond's obsession with Brown, Peterson recalls that seven years earlier, Ralph Bettini was the right-hand man of Grazzi, the former leader of the "big combination" now run by Brown. When Grazzi left New York for Sicily, Bettini disappeared, and Diamond speculates that he could provide useful information. Diamond locates the fearful old man, and Bettini relates that during the ocean voyage that took Grazzi to Sicily, Brown argued with his wife Alicia, a farm girl whose loathing of Brown's violent lifestyle turned her into an alcoholic. Alicia disappeared, and Bettini, afraid that Brown had killed her, jumped ship and went into hiding. After Bettini remembers that the ship's captain was named Nils Dreyer, Diamond goes to Dreyer's antique shop but Dreyer refuses to divulge any information about Brown. After Diamond leaves, Dreyer is gunned down by McClure, who is then castigated by Brown for resorting to violence when he was instructed only to bring Dreyer in. The next day, Diamond searches Dreyer's safe-deposit box and finds the negative of a photograph of Alicia with Brown and Grazzi, as well as a notation to refer to the 15 November 1946 log entry of the S.S. Grazzi . Diamond rushes to Dreyer's shop, where he finds that Brown has obtained legal ownership of Dreyer's papers and has burned the log. Diamond then finds Susan at a concert hall, where he gives her the photograph of Alicia and begs her to leave Brown before her life is endangered. After Alicia confronts Brown with the photograph, he gives her a recent picture of Alicia that he asserts was taken a month earlier on Grazzi's estate. Later that night, while Rita waits for Diamond in his apartment, Brown orders Fante and Mingo to kill the policeman, and the two hoods shoot through the door and kill Rita without ever seeing her. Diamond is paralyzed with guilt over the tragedy until Susan comes to his office soon after with the new picture of Alicia. Police technicians examine the photograph and learn that instead of being taken in Sicily, it was taken at an upstate sanitarium. McClure follows Diamond to the sanitarium, where he finds Alicia, who was committed by Brown just after Grazzi left the country. Although Alicia claims that she does not know Brown, McClure rushes back to Mingo and Fante, and tells them that Brown killed Grazzi and has been pretending that Grazzi is alive in Sicily so that the other syndicate members will follow him. McClure tells them that if they kill Brown now, they will become the new leaders. He then drives Brown to a private airport, where Fante and Mingo are supposed to gun him down, but instead, the two hoods aim their guns at McClure. McClure begs for his life, but Brown merely removes his hearing aid before he is shot to death. At Diamond's office, Alicia refuses to talk, even when Susan states that she will be testifying against Brown. Susan breaks down in tears upon seeing a photo of Rita's bullet-riddled corpse, and Alicia finally agrees to testify. As she is leaving the office, however, she sees Brown waiting for her and goes into a state of shock. Soon after, McClure's body is found, and Diamond realizes that Brown is becoming careless. Mingo and Fante hide in the hotel cellar for two days, until one evening, Brown brings them food and a box of money. When Fante opens the booby-trapped box, it explodes, killing him and mortally wounding Mingo. Diamond arrives before Mingo dies, and the gangster, furious over Fante's death, implicates Brown. Before Diamond can find him, though, Brown kidnaps Susan and escapes to the airport. Diamond follows them, and with Susan's help, captures the now-cowering Brown. Weary yet content, Diamond and Susan walk off together into the fog.