After training at the dolphin reconnaissance school, the dolphin Tristan was sent on an important mission to a secret military base of the USSR to obtain information about the Russians' new strategic weapon - a submarine that transforms into an airplane. After receiving a message that an unidentified object was discovered in Soviet territorial waters, moving towards a secret base, the Soviet command ordered to urgently send an experienced special agent, the dolphin Isolde, to intercept it.

Directors: Vladimir Tarasov, Rasa Strautmane, Aleksandr Mazaev, Aleksandr Gorlenko, Elena Prorokova