In 1888, all of England is horrified by the gruesome murders of streetwalkers in the Whitechapel district of London. One night, even though three thousand policeman are patroling the area, the killer, dubbed "Jack the Ripper," claims his fourth victim. That same night, William Harley, an unsuccessful businessman, and his wife Helen are surprised by a visit from Mr. Slade, who is interested in their rooms for rent. Slade expresses dismay at the parlor's drawings of oldtime actresses, but is interested in the attic, which he says will be useful for experiments. Explaining that he is a pathologist who keeps irregular hours, the soft-spoken Slade offers Helen a month's rent in advance. Helen, curious about Slade's aversion to actresses, tells him that her actress niece, Lily Bonner, is about to open in London after a successful run in Paris. On the night of Lily's opening, the Harleys' maid, Daisy, is hesitant to attend, as she does not want to walk home alone, despite the presence of an additional thousand policeman to the area. Slade then meets the beautiful Lily, who is intrigued by the shy, lonely lodger. Later, as Lily prepares in her dressing room, she is visited by faded actress Annie Rowley, who admits that she now works in a brothel. Although Lily wants to help Annie, she disappears, and Lily goes onstage, where her revealing dances are welcomed with applause. After the show, Lily is visited by Scotland Yard inspector Paul Warwick, who informs her that Annie has become the Ripper's latest victim. The police believe they have a lead, however, as a man wearing an ulster and carrying a small black bag was seen near Annie's body. The following morning, Warwick visits the Harley home to see Lily, with whom he has become infatuated. When he is introduced to Slade, however, Warwick takes an instant dislike to him, and Slade returns his antipathy, for both men are jealous of the other's interest in Lily. Warwick expounds his theory that the Ripper is left-handed and is a maniac who kills at random, but Slade replies that he will never catch the Ripper, as he is only doing what he has to because of who he is. Soon after, Helen becomes uneasy when she discovers that Slade has burned a small black bag, although William shows her that he has hidden a similar bag of his own because any man carrying such a bag is under suspicion. The following day, Slade has tea with the flirtatious Lily, who kisses him, much to his surprise and delight. Slade turns away, however, when she inquires about his dislike of actresses. Slade then reveals that his mother was an actress and that her extraordinary beauty hid "the wretched heart of a Jezebel," and that after she left his father for a younger man, he died a drunkard. Slade relates that his mother eventually became an alcoholic streetwalker and died on the streets of Whitechapel, and Lily is surprised by the depth of his conflicting feelings for her. Their discussion is interrupted by Warwick, who invites Lily to tour Scotland Yard's Black Museum, which contains artifacts of notorious killers. Slade accompanies them and is disgusted by Warwick's attitude toward the artifacts, which he proudly regards as trophies. Warwick theorizes that the Ripper has broken his pattern of killing every five or six days, and that the compulsion to kill must be building within him. Angered by Warwick's professed expertise about the Ripper, Slade predicts that he will strike that night. A few hours later, Irish immigrant Mary Lenihan is murdered, and soon after, Lily awakens at home and finds Slade burning his ulster. Although the stains on his coat look like blood, Slade asserts that he is burning the ulster to prevent contamination due to a bungled experiment. The next morning, a nervous Helen wants to alert the police about Slade's strange activities, but William refuses to listen and assures Slade that he is welcome to stay. Slade then tells Lily that he will be attending her show that evening, although he jealously notes that Warwick will also be coming. Meanwhile, Warwick, suspicious of Slade, confirms that he is a pathologist at the University Hospital, and later that night, arrives at the Harley home to escort Lily to the theater. Before Lily comes downstairs, William reluctantly apprises Warwick of Slade's eccentricities, and the inspector searchs Slade's room, in the hopes of obtaining a fingerprint to compare with one found at the latest crime scene. Lily is outraged when she finds Warwick snooping through Slade's belongings and stealing a portrait of his mother, and berates him. At the theater, Slade grows increasingly distraught by the lascivious stares Lily's dancing draws, while at Scotland Yard, Warwick is frustrated in his efforts to match Slade's fingerprints to those of the Ripper. Warwick's sergeant, Bates, cracks the case, however, when he recognizes the portrait of Slade's mother as Anne Lawrence, the Ripper's first victim. Rushing to the theater, Warwick and his men search for Slade, who is with Lily in her dressing room. Slade begs Lily to go away with him, far from the stares of other men, but the nervous Lily states that she would never give up acting, despite her fondness for him. In anguish, Slade states that he must cut out the evil from her beauty and holds a knife to Lily's throat. Lily gasps that Slade cannot hurt her because he loves her, and as Warwick and his men pound on the door, Slade drops his knife and escapes through the window. After a long chase through the streets of Whitechapel, Warwick reaches the River Thames, into which Slade has disappeared. Although Warwick searches for the killer, it becomes apparent that he has drowned, and the inspector surmises that the river is not as deep and dark as where Slade is going.