In the earliest books, Fu Manchu is an agent of the secret society, the "Si-Fan". and is the mastermind behind a wave of assassinations of Western imperialists. In later books, he seeks control of the Si-Fan. The Si-Fan is largely funded through criminal activities, particularly the drug trade and white slavery. Dr. Fu Manchu has extended his already considerable lifespan by use of the "elixir vitae" (elixir of life), a formula he spent decades trying to perfect.