Foreseeing the destruction of Earth, humanity builds a multi-generational
starship called
Earthship Ark, wide and long. The ship contains dozens of
biospheres, each kilometres across and housing people of different cultures. Their goal is to find and seed a new world of a distant star. In 2385, more than 100 years into the voyage, an unexplained accident occurs, and the ship goes into emergency mode in which each biosphere is sealed off from the others. In 2790, 405 years after the accident, Devon (
Keir Dullea) a resident of Cypress Corners, an agrarian community with a culture resembling that of the
Amish, discovers that his world is far larger and more mysterious than he had realized. Considered an outcast because of his questioning of the way things are, especially his refusal to accept the arranged marriage of his love Rachel (Gay Rowan) to his friend Garth (
Robin Ward), Devon finds out that the Cypress Corners elders have been deliberately manipulating the local computer terminal, which they call "The Voice of The Creator". The congregation pursues Devon for attacking the elders and stealing a computer cassette on which they have recorded their orders, and its leaders plot to execute him, but the elderly Abraham, who also questions the elders, gives Devon a key to a dark mysterious doorway, which Abraham himself is afraid to enter. The frightened Devon escapes into the service areas of the ship and accesses a computer data station, which explains the nature and the purpose of the Ark and hints at its problems. When Devon returns to Cypress Corners to tell his community what he has learned, he is put on trial for heresy and condemned to death by stoning. Escaping on the night before his execution with the aid of Garth, Devon convinces Rachel to come with him, and Garth pursues them. When Rachel refuses to return with Garth, he joins her and Devon. Eventually they make their way to the ship's bridge, containing the skeletal remains of its crew. It is badly damaged, and its control systems are inoperative. The three discover that the Ark is on a collision course with a
Class G star
similar to the Sun and realize that the only way to save the Ark and its passengers is to find the backup bridge at the other end of the Ark and to reactivate the navigation and propulsion systems. Getting to the other end of the Ark, which is potentially hundreds of miles away, involves traveling through the many disparate communities on the starship, some of which are hostile to outsiders, and all of which have different social customs, belief systems and law enforcement practices. Generally, each episode will have Devon, Rachel, and Garth encountering a new society or group as they work their way through the ship. Occasionally, they are aided (or hindered) in their travels by the ship's frustrating and only partially-functioning computer system interface, known as Mu Lambda 165 (portrayed by William Osler, who also provided the opening narration for each episode). == Development and production ==