Summer 1944. Soviet sailors sink a German submarine in the Baltic Sea. It has three brand new German torpedoes on board. The Nazis understand that the Russians will soon thoroughly study these torpedoes, and they send their agent to Leningrad to destroy them. Sergei Saburov and Elena Soley must neutralize the German saboteurs and identify the "mole" in their immediate circle, as well as save their love, which is threatened by the sudden return of Saburov's wife.