This chapter explores how Bruno Pontecorvo's presence in Russia was far more decisive than any information that Soviet intelligence had managed to harness earlier on, and examines the proliferation of accounts about him. It shows how the withdrawal of information from the public domain featured as a key tool of nuclear diplomacy. The chapter illustrates how the utilization of secrecy has impacted the current understanding of important transitions in nuclear science and security. Secrecy was employed in the Pontecorvo case to cover issues that had major implications for the handling of national and international affairs. The Pontecorvo affair reported that the mysteries connected with the spread of nuclear and other military technologies and the unlawful transfer of scientific information cannot be immediately understood, except for some marginal aspects. Revealingly, the new documents on this affair indicated that an intelligence program aimed at tackling atomic espionage may have never really existed.