It is not merely the best science fiction currently on television; it is also the best drama, and best military show.

I don't know if it'll be possible to jump in at the start of the second season (which starts on Friday, woo-hoo!) and have any idea what's going on, though. Rather than relying mostly on episodic stories like most shows, they've got dozens of plots interleaving throughout the whole thing -- kinda like 24, but not stretching to hit you in the gut every week.