Foreseeing the destruction of the Earth, humanity builds a multi-generational starship called Earthship Ark, 80 kilometres (50 mi) wide and 320 kilometres (200 mi) 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. More than one hundred years into the voyage, an unexplained accident occurs, and the ship goes into emergency mode, whereby each biosphere is sealed off from the others.
Four hundred and five years after the accident, Devon (Keir Dullea) a resident of Cypress Corners, a conservative 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 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 that explains the nature and purpose of the Ark and hints at its
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 and reactivate the navigation and propulsion
systems. Occasionally, they are aided by the ship's partially
functioning computer system.