This paper is divided in two parts, one theoretical and one textual and historical. The first one analyzes the seminal proposal of a “conspiracy theory of society” made by Karl Popper in 1945 and some dictionary definitions. A general scheme of conspiracy theory is proposed and then developed in semiotic terms, showing that every conspiracy theory is linked to a second conspiracy, that of the “theorist”. In the second part, anti-Semitism is presented as the prototype of this double plot situation, with the help of some Biblical texts: from many centuries ago until now, haters of Jews have used the theory of a potential Jewish conspiracy in order to attempt at eliminating them. Five theses about the link between conspiracism and anti-Semitism are presented in the conclusion.