In the final days of the Civil War, a Union captain, a freed slave, a reporter, and a family of man, wife, and son, escape from the Confederates in a stolen balloon which drifts across America and finally deposits them on a Pacific island.
A blocked water supply causes stress for the castaways, and Harding begins to show signs of paranoia and delusion. Herbert finds on the beach a letter written in Latin containing the word "Nemo."
Harding and company learn their island lies far from any mainland, is volcanic in origin, and seems prone to earthquakes. They also come to believe it's inhabited by some sort of force or power characterized by a green light.
Pirates land on the island, taking the castaways by surprise. The pirates seek to recover buried treasure and their arrival prompts the release of new and disturbing information about the hermit, Ayrton.
A rockslide of suspicious origin partially buries Gideon. To free him, Harding creates an explosion using improvised gunpowder. Later, Neb stumbles into a cave which the castaways decide to turn into a residence: Granite House.