As I mentioned in a previous post, BSG is about the survival of humans as they eke out a living in the most hostile of environments: space. In this reality, humans created cylons, robots that eventually rised up against humans and started the human-cylon war. Cylons are banished from the human planet, but return after a few decades to decimate the human population. Less than 50,000 people are left after cylons annihilate the planet, and left to try to survive in space on the ships used for luxury living, waste compacting, mining, and military protection. The Battlestar Galactica is one of the oldest ships in the "fleet," having been used during the human-cylon war, and was scheduled to be retired on the day of the cylon attack. The battlestar is used as the new military base, and most of the main plot points happen on this ship.
The show is four seasons long, and it's a brilliantly thought-out plot. Yes, it's a sci-fi show, but everything is grounded in reality; no aliens or space beings pop up. Cylons are made by humans, so it makes sense that their stories are intertwined. There are surprises throughout the series starting with the second episode, so I won't go into too much detail into the actual plots of the show.
Obviously, the biggest plotline of the show is whether humans can/will survive in space. Throughout the seasons, bigger social issues arise: what rights, if any, do non-humans have in society? What makes a person/thing human? Should abortions be allowed if the very survival of humanity at stake? Can democracy thrive in such a concentrated space? You'll watch some episodes and just be itching for discussion afterward. In addition, there's the whole personal relationship aspect of the show; when there are less than 50k people left in your civilization, it's difficult to maintain the same relationship with mankind. I read articles about this show being excellent for couples to watch together, as there's a lot of political and military intrigue, along with soap opera plots with married couples and families.
Let me just close with this: I don't regret any of the 40+ hours spent in front of the TV finishing this epic story.
So say we all.