Nelson and Crane go undercover and risk their lives in order to bring a Communist defector back to the United States. But the man is injured in the attempt, and his lover is left behind. He won't speak to the agent waiting for him on Seaview (actually a double agent) until he is reunited with his Anna. But even Anna is not all she seems

An old friend of Nelson's comes aboard Seaview to continue his research into the giant whale that destroyed his ship, crippling him and killing his son. His real motives, however, have more to do with vengeance than science, and Crane becomes concerned that the Admiral is caught up in his friend's obsession.