At the dawn of the 30th Century dolphins and humans had been together in a cross-species society for 500 years. Together they had set out to explore space offering peace and friendship to all who would welcome it. But space had its dangers; a violent species known as the Foe decided to conquer Earth. However the dolphins and humans drove them to the brink of defeat and so the Foe sought vengeance on Earth. The few caretaker dolphins who had been left behind on Earth were not entirely defenseless; they were protected by their creation called the Guardian—a gigantic sentient being made of a crystalline substance which projected a forcefield over the entire planet. Undeterred the Foe made suicide attacks on the field searching for a weak point.