At Pearson's Bookstore in London, American manager John Harman informs his secretary, Stella Tracy, that he has finally received the insurance check that will pay for a clinic visit for his ailing wife May. Meanwhile, on the sales floor, Clive Oliver chastises young clerk Ruby Bruce for her perpetual tardiness. In retaliation, when she sees patron Jeff Hart steal a book, she does not report him but instead accepts his invitation to the Blue Club that night. There, Jeff charms Ruby to the point that she begs him to let her meet him the following evening. At the shop the next day, however, John asks her to stay late to help him catalog some new books. After everyone else leaves, Ruby changes into her seductive evening clothes and joins John in his office, where she notices the insurance check on his desk. She tears her sleeve while moving books, and when John touches the rip, she kisses him. He returns the kiss but quickly pulls away. Meanwhile, at the Blue Club, Jeff and his ex-girl friend, Vi, discuss his recent release from prison, and Vi grows jealous when Ruby arrives. The shop girl tries to impress Jeff by claiming her boss manhandled her, and, intrigued, Jeff punches her arm in order to leave a bruise and then coaches her to blackmail John by claiming that he attacked her. The next day, Ruby demands money from John, but after he ignores her, Jeff insists that she write a letter to May and show John. Although he sees the letter, John still does not respond, so Jeff mails the letter without Ruby's knowledge. The next afternoon, John is informed that May died while struggling to leave her bed to burn an anonymous letter. He then calls Ruby into his office and accuses her of writing the letter, and although Clive cannot hear them, he sees John grab Ruby's arm roughly. Ruby races to Jeff and tries to back out of his scheme, but he threatens to turn her in and sends her to the shop that night. There, John is working late in order to avoid going home, and when she once again pressures him for money, he throws bills at her in frustration. She takes the money downstairs to the locker room, where she is surprised by Jeff. When she yells, he puts his hand over her mouth, asphyxiating her. John looks downstairs but, seeing nothing, leaves. The next morning, Ruby is reported missing, and Clive realizes that although she left with her umbrella the previous day, it is now at her desk. He finds one of her shoes and then, learning that John worked late the previous night, calls the police with his suspicion that John murdered Ruby. Soon after, John moves a large crate of books to his house, and upon opening it, finds Ruby's body. Just then, the police knock at his door, and in a panic he races out the back door. Now in hiding, he asks Stella to meet him and, by piecing together various facts, they finally deduce that someone must have pushed her to blackmail him. Suddenly remembering that he saw some books moved in the locker room, John realizes that he must return to the shop to find vital clues and clear his name. Before they part, John confesses his love to Stella, who kisses him. She then asks Clive to help sneak John back into the shop, and although Clive at first declares his love and refuses to help, he eventually gives in. Meanwhile, Jeff gives Vi money to buy cigarettes, and when she pays, the bill is immediately recognized as part of John's insurance payment. At the police station, Vi admits that Jeff is staying with her and then reacts jealously to a picture of Ruby, tipping off the inspector that Jeff is involved. That night, Clive helps John sneak into the shop locker room but then calls the police. The inspector, however, now suspecting Jeff, helps John search the room for clues. Meanwhile, Stella remembers the name of the Blue Club and rushes there in secret. She finds Ruby and Jeff's names in the register and asks to meet Jeff. Pretending to be a friend, Jeff brings Stella to Vi's, where he forces her inside and attempts to strangle her. Before he can finish, however, he hears the inspector and John coming to question him. Jeff starts a fire in the apartment to keep them out, but John rushes into the flames and tackles Jeff. While Jeff is being arrested, John takes Stella into his arms.