As in the original, there was a prior war between the humans and Cylons. Galactica fought that war with the other battlestars in the fleets and the war ended, with a shakey truce between the two.
The main spine of the plot is the same as the original: Cylons engage a sneak attack after 20+ years of being gone and the humans have become over-confident, which is their undoing.
Problem is, one of the twelve council members is a traitor who has used the 'peace process' as a ruse to render the colonies helpless. (Who might that be in today's U.S. Senate?)
The other problem (for the bad guys) is that Adama smelled a rat and did not stand down. That is why he and his crew and a rag tag bunch of fleeing humans survived to eventually make it to the lost 13th colony (Earth).
Which would be good except the that the Cylons can't be very far behind -8^).