In 1958 a British serviceman, based near the Woomera rocket range in South Australia, passed secrets to the Soviet Union. They concerned the joint Anglo-Australian guided missile project. Recently-released archival files reveal the intense anxiety, bordering on panic, that this security breach provoked in Canberra and London. The article places this reaction against the background of a long-term quest by Britain and Australia to convince the United States to restore wartime co-operation in the field of atomic technology and lift its embargo on the transmission of classified information. By unraveling, for the first time, the story of the Woomera spy case, the article illuminates issues of security, defence preparations and Anglo-Australian relations.