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, 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. 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.
