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Paul Madvig, a tough political boss who hails from "the wrong side of the tracks" and is rumored to be a crook, decides to back gubernatorial reform candidate Ralph Henry after he falls for Ralph's beautiful daughter Janet.  When Henry Sloss, one of Paul's assistants, protests that any association with Henry will denude Paul's power, Paul angrily throws the upstart through a glass window.  Paul's best friend and advisor, Ed Beaumont, also warns the stubborn Paul against supporting Henry, stating that the "key" to Henry's house that Paul insists he has in his grasp may be a "glass key" that will break off in his hand.

Later, gangster and gambling house proprietor Nick Varna visits Paul in his headquarters at the Voters League and demands that he provide his nightclubs with the protection against police interference for which he has paid.  Paul warns him that he is cleaning up the town and immediately calls the police chief to shut down Varna's club for good.

That night, Paul dines at the Henry home but Henry's disreputable son Taylor refuses to join them and frustrates his father by leaving to go drinking and gambling.  When Ed stops by, Janet flirts with him and expresses her disbelief that he takes Paul seriously.  A loyal friend, Ed rebuffs her and advises her that Paul is always straightforward.  Later that night, Paul's eighteen-year-old sister Opal borrows $500 from Ed, which she gives to Taylor so that he can make a payment to Varna for his huge gambling debt.  Ed forcibly drags Opal from Taylor's apartment to her home, where she argues with Paul about her relationship with Taylor.  Shortly afterward, Opal calls Ed in hysterics and warns him that Paul has gone to see Taylor, and that she fears he will kill him.  Ed goes to investigate and finds Taylor dead on the sidewalk outside his house.  Although Paul maintains that he is innocent, suspicion of murder falls on him, and Ed becomes determined to clear Paul's name.

At the funeral for Taylor, Varna tells Janet that he and the newspaper The Observer have evidence to convict Paul.  District Attorney Farr and Ed then start receiving anonymous notes which suggest that Paul is guilty.  Although Farr works under Paul's command, public pressure to solve the crime prompts him to rebel.  Paul, meanwhile, grows resentful of Ed's advice that he make peace with Varna, and he and Ed have a falling out.  Janet seeks Ed's help in finding her brother's killer, but even though Ed grudgingly admits that he likes Janet, he accuses her of "slumming" and refuses to help her.  Ed tries to advise Paul again that he is being outsmarted by his enemies, but the friendly talk turns into a fistfight and the men part company.

When Varna hears about the fight, he offers Ed $20,000 and stewardship of a casino if he will give The Observer information with which to frame Paul.  Ed rejects his offer and is taken hostage by Varna, who has his thugs, Jeff and Rusty, beat him.

In spite of several days of brutal beatings, Ed outsmarts Jeff and Rusty and escapes by starting a fire, but crashes several stories through a glass skylight.  Before his final lapse into unconsciousness in the hospital, Ed warns Paul that Varna is bringing in Sloss, who claims to be a witness to Taylor's murder, to testify against him.  After several days of extreme fever, Ed recovers and learns that Janet and Paul have become engaged.

Ed becomes alarmed when Paul mentions that Opal has taken a mysterious trip to the countryside, and after he learns that Clyde Matthews, publisher of The Observer, has a house there, Ed leaves the hospital and goes to the country house.  There he finds a gathering including Opal, Varna, Jeff, Rusty and Matthews and his wife Eloise.  Convinced that her brother is guilty, Opal has given a statement to Matthews to publish, which verifies that Paul pursued Taylor after they argued.

Hoping to shake up the group's complacency, Ed tells Eloise that Varna intends to call in the mortgage on The Observer after Matthews publishes Opal's damning article, which will force the paper into bankruptcy.  Disgusted that she has spent five years with a loser, Eloise then seduces Ed.  Distraught, Matthews is found dead after apparently committing suicide and Ed burns the suspicious-looking note which names Varna executor of his estate.  Just as Rusty and Jeff are about to beat Ed, Paul appears and knocks Jeff out.

Varna leaves, and Ed advises Paul to get a judge to appoint immediately an administrator for the Matthews estate who will cancel Opal's story.  Paul then captures Sloss and plans to hide him, until a mysterious gunman shoots Sloss.  Paul finally admits to Ed that when he argued with Taylor, he hit him, causing Taylor to strike his head on the curb and die.  Paul is arrested by Farr and held for questioning.

Ed, meanwhile, has discovered that Janet has been typing the anonymous notes.  Ed tracks the sadistic Jeff to a grungy bar and cautiously tries to lure him into linking Varna to Taylor's murder, but Varna comes in and tells him to keep quiet.  Jeff resents Varna's control over him, and after admitting that he killed Sloss, he attacks Varna and chokes him to death.  Jeff is arrested, and Ed presses Farr to make out an arrest warrant for Janet because he is convinced that she is framing Paul because she is guilty.  Farr reluctantly goes with Ed to the Henry home late that night to arrest Janet.  Before they take her away, Henry breaks down and admits that it was he who killed his son accidentally during their argument, and had asked Paul not to say anything.  Paul is released and makes plans to select a new gubernatorial candidate.

Later, at Ed's apartment, Janet confesses that she is in love with him, and although Ed reciprocates her feelings, he remains loyal to Paul and rejects her.  Paul overhears their conversation and, seeing that their love is true, takes back his engagement ring and sends them off with his best wishes.