Starlost, The: Complete Series
Threatened by a cataclysmic event, mankind builds a gigantic earthship spacecraft, the Ark, to save the last of humanity. An accident kills the crew and the air locks connecting the ship's domes are sealed. Cut off from the outside world, many communities simply forget they were ever on a spacecraft. As the centuries pass, each culture evolves in an enclosed society, each a world unto itself that is fifty miles in diameter. Unbeknownst to the Ark's inhabitants, they are drifting through deep space on a collision course with a sun.
In 2790 A. D., a young man named Devon (Keir Dullea from 2001: A Space Odyssey), a resident of a biosphere called Cypress Corners, discovers that his world is much larger than he was taught to believe and uncovers the Ark's fate. Devon and friends Rachel (Gay Rowan) and Garth (Robin Ward) must learn all they can about the Ark if they are to save it and the colonists aboard.