The Seaview is host to a joint scientific project with the USSR, to discover what destroyed a Russian deep sea laboratory. Nelson and Russian scientist Katya Markhova descend in the diving bell to investigate the wreckage when the bell is swallowed whole by an immense migrating whale. With only an hour of air left, Crane and a rescue party race against time in an attempt to swim inside the whale and attach a cable for the Seaview to pull them to safety.