April 1941. According to intelligence received by the Chekists, a fascist sabotage group plans to blow up one of Moscow's defense plants. To find out the plans of saboteurs working under the auspices of the German embassy, the NKVD sends one of its best employees, Sergei Biryukov, to the plant. He finds contact with the fascist agent Keller and finds out that the explosion is scheduled for June 21...