Louis and August Siever, twin sons of a German father and American mother, are traveling through Europe when war breaks out. August joins the Kaiser's army, while Louis, a loyal American, is trapped in Berlin for a year while he tries to prove his citizenship. After a violent confrontation with Louis, August steals his brother's passport and leaves for New York City with Gerda Anderson, a German spy. Louis also returns to the U.S., and sometime later is invited to a weekend party on Long Island by his wealthy friends, the Waynes. When August and Gerda learn of the event, they rent a nearby house and invite all of the Waynes' guests, including Louis, to a "mystery" party. Upon their arrival, the women are held prisoner and ransom notes are sent to their husbands via carrier pigeon. Shirley Wayne and Louis track down and are captured by the kidnappers. Meanwhile, Mortimer Eddington, an amateur detective, devises a method to trace the pigeons back to the house. Before Shirley and Louis come to harm, police officers arrive and demand the Germans' surrender.
Louis and August Siever, born in Germany of American parents, have grown to manhood in Berlin. Their environment has had an opposite effect upon the twins. Louis is strongly American while August, pro-German, has attained a captaincy in the German Army. In one of Berlin's cafés Louis has an embarrassing meeting with Gerda Anderson, who is in the employ of Germany's secret service. She is at another table in company with Capt. August Siever, when she crosses to the table occupied by Louis and the Waynes and insults the twin of the man who is infatuated with her. Louis shows his resentment by thrashing his brother then and there, knowing that his act will mean trouble for him. Louis hurries to his apartments and while packing is assaulted by his twin brother and rendered unconscious. Capt. August takes his brother's passport and goes to America with Gerda Anderson, who has been ordered to engage in her work as a German spy in the United States. Louis goes to America where the Waynes have preceded him. When August reads that the Waynes are giving a weekend party at their home in Long Island, he conceives a blackmailing scheme and Gerda joins him in his plan. They lease a house near the Waynes, and invite the Waynes and their guests to a "mystery party," promising to break the monotony of successive days in the country through an innovation that embodies mystery with daring. Louis Siever, who is at the Wayne party, leads in the impulse of many guests to accept, and in the end the party that accepts the invitation include all of the women and many of the men, excepting the husbands, who elect to stay at home and play poker. Once the Wayne party find themselves in the "house of mystery" they discover they are prisoners. Capt. August and Gerda now cause to be delivered at the Wayne house baskets addressed to all of the husbands and each containing a carrier pigeon. The winged messengers bear a note tied to their legs, demanding ransom for the imprisoned women. Mortimer Eddington has long believed himself a detective who has missed his occupation. He is a chemist, and starts at once a plan to locate the "house of mystery" and rescue the prisoners. Louis Siever and Shirley Wayne are meanwhile keeping their eyes open and trying to find a way for the imprisoned party to escape. Shirley discovers a means to reach Capt. August and Gerda, who are operating in a room upstairs. They overpower Shirley when they discover that she has intruded. Louis discovers her absence and undertakes to trace and rescue her. Meanwhile the amateur detective has been doing some good work. He has devised a scientific method of tracing the return flight of the pigeons as they are separately released from their baskets, and has succeeded in giving the police the trail. At the moment the officers are closing in on the "house of mystery," Capt. August and Gerda discover that their game is up. They decide to avenge themselves upon their innocent captives by blowing up the house with chemicals, but fate turns a strange trick and they are, themselves, the only victims of their own "frightfulness."
