Sandra Carpenter is an American who came to London to perform in a show, but now is working as a taxi dancer. She is upset to find out that friend and fellow dancer Lucy Barnard is missing and believed to be the latest victim of the notorious "Poet Killer," who lures victims with ads in newspapers' personal columns and sends poems to taunt the police. Scotland Yard Inspector Harley Temple believes the killer to be influenced by the 19th-century French poetCharles Baudelaire. He asks if Sandra would be willing to work undercover to help find her missing friend and the killer. He sees first-hand how observant she is and gives her a temporary police identification card and a gun. Sandra is asked to answer personal ads, watched over by an officer bodyguard, H.R. Barrett. By coincidence, she meets the dashing man-about-town stage revue producer Robert Fleming. In the meantime, Sandra answers an ad placed by Charles van Druten, a former fashion designer who is now mentally imbalanced. Barrett has to come to her rescue. She also needs to be saved, this time by Fleming, from a mysterious figure named Mr. Moryani. He apparently lures young women to South America by offering them a promising opportunity while, in reality, wanting to recruit them for forced slavery or other forced services. Fleming shares a stately home with Julian Wilde, his business partner and friend. Fleming ultimately does win Sandra's heart, and they become engaged. Inspector Temple thanks her for her efforts and even agrees to come to their engagement party. During the party, however, Sandra accidentally discovers incriminating evidence in Fleming's desk, including a distinctive bracelet worn by her friend Lucy and her photograph. Fleming learns that Sandra was an undercover police agent and is placed under arrest. Circumstantial evidence mounts up, his typewriter is identified as the one used for the poems, although he adamantly denies any involvement in the crimes. Sandra still loves him and believes him, but Scotland Yard do not. Fleming refuses to see Sandra, believing she only pretended to be in love with him in order to trap him. Lucy's body is found in the river. Wilde assures his incarcerated friend that he will hire the best attorney and do everything possible to clear him. It occurs to Inspector Temple that it is Wilde who fancies poetry and more likely to be the killer. Temple confronts Wilde, but has no proof, and then learns that Fleming has confessed to the murders. As Wilde prepares to flee the country, he is visited by Sandra. He is secretly obsessed with her, just as he was with the other women he abducted. Wilde at first expresses his desire for Sandra, then removes his scarf and tries to strangle her. Scotland Yard's men break through the windows to rescue her just in time. It is revealed that Fleming's confession was faked, part of a scheme by Sandra and Temple to trap Wilde. Fleming is set free, and he and Sandra make up and toast to better days ahead.
Film critic Dennis Schwartz gave the film a mixed review, writing, "The flawed film never settles into a dark and sinister mood but succeeds only in keeping things tension-free and lighthearted with continuous breezy comical conversations as Ball does a sturdy Nancy Drew turn at sleuthing with her comical detective partner Zucco. It can't quite measure up to compelling film noir, but it's pleasing and easy to handle despite everything feeling so contrived and confining."
