The book "The Spy Who Changed the World: Klaus Fuchs, Physicist and Doppelganger of the Soviet Agent" tells about the life and work of Klaus Fuchs, a famous physicist who worked in Great Britain on the atomic bomb during World War II. Having turned out to be an accomplice of Soviet spies, he gave them important information about nuclear technologies, which allowed the USSR to develop its atomic weapons. The book reveals in detail the complex motives and mental divisions that accompanied the actions of Klaus Fuchs, as well as the consequences of his activities for world politics. The author explores in depth his sense of commitment to the scientific community and the ideals of communism that played a role in his choice to become a spy. This book offers the reader a new perspective on one of the most famous doubles in the history of espionage and the atomic age.