The novel is constructed of four concentric layers, one within the other. In the outermost layer, Robert Walton, in his letters to his sister, describes his voyage towards the North Pole and his encounter with Victor Frankenstein. In the main, middle layer, Frankenstein tells Walton how he created the monster and abandoned it in disgust, how it avenged itself by murdering all those he loved and how he finally turned and pursued it. In the very center, the monster (or creature) himself describes the development of his mind after the flight from the laboratory and his bitterness when men reject him. And the other layer is composed of the letters Robert Walton’s sister sends him while he is away on his journey.