Henry S. Koser

The intrepid reporter and amateur sleuth Joseph Rouletabille becomes involved in the investigation of a hideous crime committed in a room painted yellow, whose door was locked from the inside…

During World War I, Herr Dresser, a German-American professor from West Hoboken, New Jersey, invents a "mustache fixer," which stiffens the whiskers, making the wearer look very fierce. Much to the consternation of Dresser's daughter Elsie, a patriotic American, Kaiser Wilhelm calls them to Berlin to begin mass production of the tonic for the German army. Elsie's boyfriend, Teddy Swift, is particularly disturbed by this turn of events and decides to earn enough money to follow her to Germany. When the United States joins the war, Teddy is among the first to enlist, and soon he finds himself in Berlin trying to help Elsie escape from prison. After several narrow escapes, the two make their way to the mustache factory and blow it up. Brought before the Kaiser, they are rescued when American troops storm the palace, and the Kaiser loses his mustache and the war.