The Duke of Harrington, who is engaged to the Baroness Rauxel, a millionairess, is in financial difficulties, and is beset by his creditors for immediate settlement of their claims. In agony of mind, he cannot conceive how his honor can be saved, until he recalls a dangerous and daring "Fire Act," which he witnessed years ago in India, and which he thinks might be turned to good account as a music-hall "turn." Disguising himself, he secures a two months' contract to perform such act, at a salary of £1,000 per month, under the pseudonym of "The Great Unknown." While signing the contract he is introduced to Lola Vandissen, an actress who evinces great interest in his forthcoming performance. The first night arrives, and the thrilling "Fire Act" is received with tremendous ovation from both audience and members of the company. The identity of "The Great Unknown," however, remains a secret, and he continues his attentions to the Baroness, who, hearing of the wonderful performance, asks him to take her to the theater to witness it. He cannot refuse, and conducts her one evening, but arranges with an attendant to call him away on "business" just before his turn is due. This is done, the "Fire Act" is given, and he returns to the Baroness. While behind the scenes, however, he is a witness of Lola's jealousy, who creates a scene when she discovers her fiancé in the act of kissing another artist. "The Unknown" calms Lola's temper, and her interest in him is further aroused, until she determines to discover his identity. She encounters him entering his dressing room without his disguise, but he eludes her, although she waits for him to emerge after his act. It is apparent, however, that Lola and "The Unknown" are attracted to each other, although Lola creates another scene with her fiancé on account of his attentions elsewhere. One night, during the performance of the "Fire Act" there is an accident to "The Unknown" apparatus. The theater catches fire, and in the panic which ensues, and which is most realistically portrayed, a performing baboon, which has been continually tormented by Lola's fiancé, breaks loose from its cage. The "Unknown," seeing the animal at liberty, takes refuge in the corridor of the theater, from where he is a witness to Lola's fiancé being overcome by smoke, and falling senseless to the ground, and to the baboon, who has been trained to use a knife, finding him and instantly stabbing him to death, afterwards escaping through a window. Lola, who has previously been rescued from the flames by "The Unknown," rushes along the corridor and discovers the dead body of her fiancé. She is horror-stricken and falls on her knees, staining her dress with his blood. She is discovered by the other artists and attendants, and promptly accused of the murder, and although "The Unknown" is aware of the actual culprit, he is afraid to speak for fear of betraying his identity and thus jeopardizing his engagement with the Baroness. Lola is brought to trial. "The Unknown" engages the best known counsel to conduct her defense, and, by an irony of fate, is chosen as foreman of the jury. The day of the trial arrives, and the court scene is in harmony with the gorgeous staging of the whole production, magnificently represented. Evidence is given that Lola was found beside the dead man's body, and that her handkerchief and dress were besmeared with blood, while several witnesses testify to the scenes which aroused her jealousy at the theater. Notwithstanding all the efforts of the Duke, she is found guilty, but before sentence is pronounced, the Duke's conscience triumphs and he asks to be allowed to speak. Amid breathless excitement he explains how he, the Duke, is the famous "Great Unknown," and tells the court how he witnessed the murder by the Baboon. Among the audience in the court is the Baroness, his fiancée, who, at his explanation, leaves the court in disgust. The verdict is reversed, Lola is released, and on the following morning she is seen entering her rooms accompanied by the Duke. She thanks him for saving her life, and tells him of her intentions of going abroad. This the Duke will not hear of, and instances his devotion in having announced his identity in order to save her life. Lola needs little convincing of the depth of the Duke's love for her, and the story ends with the lovers in close embrace.
