 The eponymous heroine, Isabel Thorne, is a young woman, half British, half Italian, who works for the Italian Secret Service.She has been commissioned to bring about the signing of a secret contract, in the capital of the enemy, by representatives of all countries involved, both European and American.Her brother, an inventor, has devised a secret weapon by which missiles can be fired from submarines (see also depth charge) which will, it is hoped, secure military domination over the rest of the world.