This is a study of a Soviet espionage mission in Sweden in 1942 under the aegis of the intelligence officer Vasilii Sidorenko. Aside from trying to reconstruct, on the basis of material filed with the archive of the Swedish Security Police (Säpo), the history of the mission, the article also addresses methodological challenges following from the fact that we must, as Soviet material remains very scarce, approach Sidorenko’s mission indirectly, through material generated by the Swedish police. The article is therefore structured in terms of three separate fields of inquiry—a first that addresses the organization of Sweden’s counterespionage with regard the Sidorenko mission, a second that addresses the organization and institutional context within which Sidorenko operated, and a third that addresses the accounts of five Swedish citizens who were suspected and later sentenced for having assisted Sidorenko to collect military intelligence in Sweden. The article concludes that the suggested framework with three fields of inquiry might be helpful in the study of offensive espionage operations in a more general sense.