In Los Angeles, while writing a feature story about telephone operators, columnist Casey Mayo sees calendar girl artist Harry Prebble sketching the operators. In vain, Prebble tries to make a date with one of the women, divorcée Crystal Carpenter, but she leaves with her two roommates and fellow operators, Norah Larkin and Sally Ellis. Prebble then cuts short a phone call from Rose, a woman he has been dating, who is almost hysterical in her need to talk to him. That night, Norah spends her birthday alone and reads a letter from her fiancé, a soldier fighting in Korea. After she reads that he is breaking their engagement, the telephone rings, and still in shock, she answers and agrees to have dinner at the Blue Gardenia Restaurant with Prebble, who thinks he is talking to Crystal. At the restaurant, as Norah drinks too much, Prebble buys her a gardenia corsage from a blind flower vendor and the restaurant entertainer sings "Blue Gardenia." Later, after taking the drunken Norah to his apartment and putting on a recording of "Blue Gardenia," Prebble tries to force himself on her. As Norah struggles, she hits him with a fireplace poker and breaks a mirror, but then faints. She runs out in haste after she awakens, leaving her shoes behind. Norah suffers a hangover the next day and can recall little of the previous evening, but after reading about Prebble's murder in the newspaper, she fears that she killed him. Police captain Sam Haynes's investigation has been stymied by Prebble's housekeeper, who has wiped off fingerprints and tidied Prebble's apartment. Haynes's only clues to the murder are a pair of women's shoes, a gardenia corsage, a lacy handkerchief and the record on the phonograph turntable, which was still spinning when the housekeeper arrived. While the police proceed with a manhunt, Casey takes an interest and shows up at Prebble's apartment, where Haynes plays the record for him. After tracing the corsage to the restaurant, Casey nicknames the killer, "The Blue Gardenia," and writes her an open letter, promising the newspaper's help in hiring the best legal defense, if she will come to him with her story. Casey is harangued by crank calls and dismisses a call from Rose, because she does not identify the shoes found by the police. Meanwhile, Norah's tension is noticed by Crystal and Sally, who assume she is grieving for her ex-boyfriend. Tormented by memory flashes, Norah tries to piece together events of the terrifying night. During his own investigation, Casey learns from the Blue Gardenia staff that Prebble was with a blonde in black satin on the night of his death. Reading the description in Casey's column, Norah burns her dress, but then, late one night, decides to contact Casey from a pay phone. After claiming to be calling for a friend, she agrees to meet him at an all-night diner, where she explains that her "friend" can only remember swinging the poker in self-defense while "Blue Gardenia" played on the phonograph. Casey finds himself attracted to Norah, but hides his feelings and urges her to have the friend meet him the next day at the diner. When Norah returns home, Crystal is waiting, having connected Norah to the now famous murder case, and the next day, accompanies her to the diner. After surprising Casey with the revelation that she is the "Blue Gardenia," Norah realizes that Casey's offer to help was a trick to get a story. Norah leaves, but is arrested by Haynes and his men, who were tipped off by the diner staff. As Norah, who now believes that Casey set a trap, is booked, Casey proceeds to the airport with his photographer, Al, for an overseas assignment. Waiting for the plane, Casey recognizes music being piped into the lounge as that found on Prebble's phonograph, then remembers Norah saying that Prebble played a different song, "Blue Gardenia," when she was there. He contacts Haynes and together they go to the record shop where Prebble bought the record found by the police. Rose, who works there, thinks they have come for her and attempts suicide. Later, at the hospital, Rose confesses that she showed up at Prebble's to talk that night and seeing Norah's corsage and handkerchief, realized he was seeing someone else. Rose explains that Prebble tried to calm her by playing the song that he bought at the record store when they first met, but she killed him in a jealous rage. Norah is released, but remains distant from Casey, who wants to make amends. However, Crystal advises him not to give up, and he feels sure that he and Norah have a future together.