The main plot is pretty standard - humans and aliens coming together against the threat of a dark alien menace.
The strength of Babylon 5, though, is the scripting and characterisation. J Michael Straczynski explores a lot of very creative avenues, and the overall plot execution is one of the best sf on TV. Add to that some great character and dialogue and what you have is a superb melting pot of entertainment. Also note that every single episode was written with a key overall plot in mind, so everything ties in at some point, and much more tightly than any other sf series I can think of.
The whole show was separated into 5 main Seasons (series):
The first season I've never actually watched properly - just a few episodes - but it is generally regarded as pretty slow paced. Even still, it sets up the foundation for everything that follows. So if you have *lots* of time to kill, then it's a good place to start.
Season 2 is where it steps up a gear - the "dark menace" finally starts to make it's appearance, and all at a time when there are incredible character plotlines taking place (such as one main character's race conquering another main character's race). There's a lot of general storytelling here, but it becomes pretty obvious that a lot of seemingly unconnection plot threads are actually coming together as one.
Season 3 is where everything really starts to hit the fan - the threads finally all start tying together, the gear goes up tempo, and there's some great writing to be had (J Michael Straczynski is one of few on TV who can actually write about time travel in a way that
works).
Season 4 - well, it all starts interesting enough. Unfortunately, the whole concept was written around 5 seasons, but the bankers told them to close in 4, apparently, so it all becomes rushed. The end is result is very unsatisfying.
Then apparently Babylon 5 was given backing for another season - but what could they write about? The story has alerady been told
and finished. So we end up with a half-assed, incomplete and really quite non-sensical Season 5 with appalling scripting and a general lack of appeal (I suspect J Michael Straczynkski thought he could get away with a 6th season).
There are spin-off TV movies and pilots - but what I've seen is pretty turd.
Overall - even though the end is crap and completely unsatisfying, it's a meal worth tasting of - because when it was good, it was one of the best sf's on TV. But when it was bad it just as crud as all the rest could get.
I've already bought and watched Season 3, but I'll buy 1,2 and 4. Will never buy 5. Season 2 might be a good place to start, but there will still be a lot from Season 1 it would help to know.
Hope that helps.