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.

Dolphines Tristan and Pavlova adopt a boy, Mikko, left behind after a shipwreck.

Depicts average routine of average Soviet woman who raises children, cooks at home and goes to work every day.