[4] In Soviet territorial waters near a secluded bay on an island (filmed in Crimea), a team conducts experimental trials of a groundbreaking gas mixture for underwater breathing apparatuses.
The trials involve the inventor of the gas, Sretensky, alongside Dr. Kushlya, Captain Second Rank Igor Belogurov, and elite divers from across the Soviet Navy.
The Soviet counterintelligence agency sends Senior Lieutenant of State Security Lyudmila Skuratova, disguised as a graduate student named Balashova, to the base where the trials are being conducted.
In a tense encounter, Yeltsov attempts to eliminate Skuratova, but she outmaneuvers him while gaining the trust of the scuba diver, who believes she is the genuine Balashova.
The film closes with a close-up of Skuratova's face, capturing the emotional weight of her ordeal and the resilience required for such a dangerous mission.