Expert forger Jim Fleg kills a guard while stealing a rare edition of Hamlet from a public library, then manufactures copies of the book for private collectors. His accomplice is Myra Blandy, a supposedly legitimate book dealer who "authenticates" the forgeries when another accomplice, Rebescu, sells them. One afternoon, Rebescu and Myra sell a copy of Hamlet to Martin Cleaver, a mysterious German. Fleg is furious when Myra informs him about the deal, as he knows that the dangerous Cleaver purchases stolen artworks for a high-ranking Nazi official. Fleg, who is obsessed with his sadomasochistic tendencies and his twisted relationship with Myra, orders her to return Cleaver's money. Myra's situation worsens when Cleaver discovers that the manuscript is a fake, kills Rebescu and demands that Myra take him to Fleg. Another complication arises when private detective Hal McByrne comes to Myra's office and reveals that he is aware of her illegal activities. Instantly attracted to Myra, McByrne agrees to protect her if she helps him catch Fleg. Fleg in turn finds out about McByrne, and Myra, desperate to extricate herself from her predicament, sets up McByrne. She tells Cleaver that she will send Fleg to the library that evening to pick up a book held under her name, but she instead sends in McByrne while she waits in the car. Cleaver and his thugs, Benson and deaf-mute Eric Pahsen, hold McByrne captive in the library and refuse to believe his protestations that he is not Fleg. Meanwhile, Fleg has learned of Myra's scheme and concocted a ruse of his own. Impersonating a police lieutenant, Fleg goes to the library, where he intends to kill McByrne. Fleg accidentally kills Cleaver instead, but quickly recovers and, using the actors he has brought along to play policemen, begins an "investigation" of the crime. He demands that the librarians and patrons remain as witnesses and that the valuable books being kept in the vault be turned over to him for police protection. Meanwhile, Myra plays Fleg and McByrne against each other, reassuring each that she is working for him. Because of Fleg's knowledge of the rare books, McByrne guesses his real identity and as he tries to escape, is pursued by Hollis, Fleg's bodyguard, and Benson and Pahsen. Myra hides the books, which she intends to retrieve later for her own gain. As the various factions conspire and pursue one another, a blackout is announced, and the library's air raid warden escorts everyone to the basement. Hollis takes advantage of the confusion to chase librarian Kay Ryan, whom Fleg suspects is aiding McByrne. McByrne shoots and kills Hollis before he can kill Kay, but is then captured by Fleg, who threatens to torture Myra if she does not reveal the location of the hidden books. Using the inter-office phone, McByrne convinces nervous librarian Miss Oval that the blackout is over, and she turns on the lights. Air raid wardens patrolling the streets are alerted and rush in to investigate. The police also arrive and soon the criminals are rounded up. McByrne explains the situation to the police and then searches for the hidden books while the captured Fleg eagerly anticipates dying in terror after his conviction. Using the Dewey Decimal system and a clue left by Myra, McByrne finds the books and returns them for a reward. Myra, afraid for her life, then pleads with McByrne to escort her home. Tired of her double-crossing, McByrne leaves Myra to fend for herself, and she is strangled by Pahsen, who has temporarily eluded the police. Pahsen is apprehended, and McByrne, both despondent and embittered by his involvement with Myra, leaves the library with Kay.