the Battlestar Galactica
three hour pilot
.. first aired on September 17, 1978
after many years ("yahrens") of war the Cylons and humans of the Twelve Colonies finally agree to sign a treaty of peace .. the two races are set to meet in a remote area of space .. the human leaders foolishly send all of their Battlestars to the meeting leaving the home colonies undefended.
needless to say the Cylons have no intention of making peace, it was simply a ploy to draw the human fleet away so the Cylons can easily destroy the colonies .. which is exactly what happens .. and to boot all of the Battlestars except the Galactica are destroyed.
the Galactica returns to the destroyed colonies and Commander Adama orders all ships to transport as many people as possible off of the planets .. the survivors then set out to find the thirteenth colony, Earth.