The four armourers in the title refer to Frau Schindler of Germany, M. Lemesurier of France, Senor Mendosa of Spain, de Castro from South America, four very successful and very avaricious armament manufacturers. They are all after a secret chemical formula now in the hands of the inventor's widow. Also after the information is an American who is using his daughter, Julia, to assist him. Going against them is Colonel Granby and his friend, John Baxter. We the readers are left until the end wondering why the formula is so important. Most important in this adventure for Granby is the fact that he meets the lovely Julia. This tale is told from the viewpoint of Baxter.