After New York police lieutenant Frank Reeves warns private investigator Bradford Galt, who is new to town, that Reeves's California friends have asked him to keep an eye on him, Brad, frustrated that his past has continued to haunt him, invites his secretary, Kathleen Stuart, to dinner. When they notice a man in a white suit following them, Brad puts Kathleen in a cab, instructing her to park by his office and follow the man after he meets with him. Brad then surprises the man in a dark corner and forces him at gunpoint to go to the office. There, Brad coerces him into revealing that his client is attorney Anthony Jardine. When the man spills ink on Brad's desk, Brad wipes it on the man's suit and sends him out, keeping his wallet. Kathleen tries to follow in her cab, but "White Suit" loses them. She finds Brad drinking in his office, and when she tells him she wants to help, he kisses her. He then warns her to leave and not get mixed up in his problems, but she insists on staying. Meanwhile, at a party celebrating the anniversary of elderly rich art dealer Hardy Cathcart and his young, attractive wife Mari, Jardine returns to Lucy Wilding, an older, married woman, the love letters he once wrote to her, having received a Van Gogh painting as blackmail from her. The next evening, White Suit follows Brad and Kathleen from a nightclub to her apartment, and after she refuses to let Brad in, White Suit tries to run him down in his car. A newsboy tells Brad the partial license plate number of his assailant, and as Kathleen and Brad wait at a neighborhood café for the police to trace the plate, he tells her about Jardine: In San Francisco, Jardine, who preyed on wealthy women by seducing and then blackmailing them, was his partner in a law firm. When Brad caught Jardine stealing the firm's money, Jardine offered to drive Brad to his house to pay back the money from his safe, but instead knocked him out, plied him with scotch, and put him behind the wheel, after which he hit a truck, killing the driver. Brad received a two-year sentence for manslaughter, but was released early for good behavior. As Brad relates his story, White Suit parks his car in front of the Cathcart Gallery, and Jardine drives off in it. Later, after Cathcart leaves to buy a painting, Mari goes to Jardine's apartment, where she convinces him to run away with her the next day. Brad, having gotten Jardine's address from the police trace, visits him, and while Mari, hiding in the bedroom, listens, he knocks Jardine out after Jardine denies hiring White Suit to follow him. Mari calls the police, and when they arrive, Jardine sends her out the back way and only gives up Brad's name after they threaten to take him in for questioning. The next day, at an exhibition, Cathcart escorts some guests to his downstairs vault to show them the Raphael that he recently bought. The guests and Mari are surprised to see that the portrait greatly resembles Mari, and Cathcart explains that he worshipped the portrait ever since he saw it, and when he met Mari, it was as if he had always wanted her, too. After the others go back upstairs, Cathcart overhears Mari and Jardine plan to leave that night, and sees their shadows as they kiss. Cathcart then finds White Suit, the man he hired to follow Brad, waiting to report that Brad roughed up Jardine but failed to kill him as Cathcart had hoped. Cathcart instructs White Suit to call Brad and, offering to sell him information on Jardine, set up an appointment at Brad's apartment. After Brad sends Kathleen to a movie, he returns to his apartment, where White Suit, who has slipped in through a window, anesthetizes him with ether. When Jardine, summoned by Cathcart, arrives, White Suit kills him with a fireplace poker, then puts the poker in Brad's hand and departs. Awakened by the sound of Kathleen ringing his door buzzer, Brad drags the body under his bed and tells her to leave. Instead, she cleans the blood from the poker and rug, and says she is hanging on to him. They try to find White Suit, but the wallet Brad seized from him turns out to be stolen. Brad awakens the next day in Kathleen's apartment, and when they spill a cup of coffee, he remembers the spilled ink on the white suit. They contact the large cleaning and dye plants to inquire about a white suit, but find no leads. Meanwhile, White Suit phones Cathcart to demand his money, and Cathcart instructs him to meet him at his dentist's office that afternoon. Brad finally tracks down the white suit and goes to the address of its owner, but discovers that the man, whose real name is Stouffer, has just left with his suitcases. A girl, however, overheard Stouffer telephoning Cathcart and gives Brad the address. At his dentist's building, Cathcart pushes Stouffer out a high-rise window, and Brad arrives just in time to witness his death and overhear the cab driver say that his bags are still in the cab. Brad steals the cab, but when he and Kathleen search Stouffer's belongings, they find no helpful information. Brad then remembers that the little girl mentioned something about "cascara at the galleries." With Kathleen's help, Brad deduces that the girl was referring to the Cathcart Gallery and goes there, just as the police, who have found Jardine's body, arrive at his office to arrest him. At the gallery, Brad meets Mari, and while they both wait for Cathcart, who has gone to the vault, he guesses that she was involved with Jardine. After he tells her that Jardine is dead, she faints. Entering the room, Cathcart orders Brad into the vault at gunpoint, planning to kill him, but Mari, now revived, shoots her husband, then throws the gun down in disgust. Afterward, Reeves makes an appointment to meet Brad the next day, and Kathleen says it will have to wait until the afternoon, as they have a date to get married at city hall in the morning.
