Victor Grandison is a New York bon vivant and host of a popular radio show in which he narrates captivating true murder mysteries. Grandison is "The Unsuspected" -- a man of unquestioned repute who is himself involved in murderous enterprises.
Grandison's assistant Roslyn is found hanging from a chandelier in Grandison's private office; however, no one is charged and the case is officially classified as a suicide.
Grandison lives beyond his means in a palatial mansion, a house-of-cards life-style made possible through his beautiful young ward and niece, heiress Matilda Frasier who is believed to have recently died in a boating accident off the coast of Brasil. Stephen Howard shows up at the mansion, claiming to have married Frasier three days before her death. His story checks out, but Grandison is suspicious of Howard, given the convenient timing of his appearance -- just prior to Frasier's estate being probated. But Howard is himself a million dollar heir from a rich family and claims he isn't interested in Frasier's money. Meanwhile, a telegram arrives at the mansion -- Frasier was rescued at sea and is alive, flying in from Brasil the next day.
Howard meets her at the airport, whisking Frasier away from reporters and photographers. Frasier is astonished to hear Howard's claim that they were married -- she remembers nothing of the purported marriage, but admits that she suffered a mental breakdown in Brasil. She had escaped to Brasil after losing her beau Oliver to a live-in relative and rival, Grandison's other niece Althea. On the drive home, Howard attempts to prove the validity of their marriage certificate, taking Frasier to the home of a believable judge who with his wife remember details of the marriage at their residence. Frasier is confused -- why does she remember other details before and after the boating accident and not her marriage to a man she can't remember ever meeting?
Grandison discovers a love note in Howard's wallet -- from Roslyn. It's apparent that Howard concocted the story of his marriage to Frasier in order to investigate the details of Roslyn's murder -- he knew she wouldn't have committed suicide. Howard confesses all to the heiress Frasier. The judge and another assistant of Grandison's agreed to the ruse in order to help Howard find Roslyn's murderer. Howard tells Frasier that Grandison is guilty and that her life is in danger, but her love and devotion to Grandison is such that she doesn't believe Howard, even though it's apparent Frasier is falling in love with Howard.
Grandison overhears his niece Althea admit to Howard that she believes Grandison murdered Roslyn because she spoke with Roslyn and heard her scream just before she was strangled. Howard shares the evidence he accumulated with the police and it's strong enough to reopen the case -- but not strong enough to charge The Unsuspected with murder.
Meanwhile, Grandison secretly tapes an argument between Althea and her husband Oliver, who was mad enough to have threatened to kill Althea. In order to eliminate a source of evidence against him, Grandison shoots and kills his niece Althea. He skillfully uses the tape recording he made of the argument between Oliver's and Althea to make it appear Oliver murdered his wife. Unaware of Althea's death, Oliver drives away from the mansion in a car that will soon have no brakes -- Grandison had severed the brake line to Oliver's car. Without brakes, Oliver dies when his car careens over a precipice. The tape recording makes it look as though Oliver killed Althea before he departed and Grandison again escapes suspicion.
Grandison arranges to have Frasier to take informal dictation for his radio show, and she writes a suicide note dictated by Grandison on the belief that it is part of his radio script. Howard and Frasier confess their growing love for each other, but Grandison hires a thug to get rid of Howard, who is carted away in a large trunk. By now, Frasier is herself becoming suspicious of Grandison but she she has a drink with him, unaware that he has spiked her cocktail with the intention of killing her for her money -- using the innocuous suicide note that Frasier had innocently transcribed. Alerted by Howard before he was knocked out and taken away, the police arrive at the mansion, just in time to revive Frasier. Howard is saved from death after a police chase with the thug hired by Grandison.
Believing that he remains The Unsuspected and that his dreams and schemes will finally be rewarded with untold wealth from Frasier's estate, Grandison appears on stage for a new radio show about yet another murder mystery. While reading from his prepared script, Grandison notices police arriving in the studio, blocking exits. He is shocked to see Frasier and Howard enter the auditorium together, arm in arm, taking seats in the audience.
Shaken, Grandison announces to his listening audience that it will be his last show -- for he is indeed a murderer and ... The Unsuspected..