Ok, for those of you who don't care about spoilers, but don't know the story, let me give you the brief overhaul.
It's a sci-fi series set in the 2260's. After a war that nearly had the human race exterminated (let's just say the Texans defending the Alamo had much better odds of survival) the enemy, the Mimbari, surrendered on the eve of their victory.
The Mimbari had been systematically destroying human civilization and colonies, down to the last man, woman, and child. They showed no mercy, for the humans attacked them without provocation. In truth it was a misunderstanding of customs upon their first contact.
The reason for the surrender... the humans never really were enlightened to, and only a few in the most secretive and upper echelons of power knew on the Mimbari side. The reason is while they are battling those human forces at the battle of the line, they decide to capture 1 foe, before they make their final decision (which was more a formality in their expectations than what eventually happened) on destroying the world.
What happened was the enemy soldier they capture, one Jeffrey Sinclair... after interrogation by them, caused a holy artifact of theirs to respond... meaning that he had a Mimbari soul... and they suspected that of their great prophet Valen. Who had predicted a coming war with the shadows.
So they surrendered... but failed to tell the reason why to their warrior caste... which eventually paves the way for a civil war.
Because of the war, the humans create the Babylon project. Essentially it's a intergalatic version of the United Nations located on a space station in neutral territory. The first 4 stations were sabotaged, attacked, or went missing within the first week (if not sooner) of it going operational. Hence the fifth station, gives the series its name: Babylon 5.
The plot and writing is rich, multi-layered, and complex. There is no black and white in this universe... just a hell of a lot of varying shades of grey.
The Mimbari form the rangers... a group of people primarily of Mimbari origin, though their are humans, and I believe eventually later on other species as well, that walk in the dark places. Doing reconnaissance on the ancient enemy, the shadows who are now beginning to re-awake.
The Ranger motto is "We live for the One, we die for the One." The One, being the current leader of the rangers.
Eventually, a human Ranger, Marcus Cole, is stationed on Babylon 5. He is... quite a card, and one of my favorite characters from any series of all time. He's sarcastic and witty, even debonair. And he falls for the Russian station Commander, Susan Invanova.
Marcus is quite the romantic... but he hides behind his humor, and tries to bring Susan out of her shell. Susan is... quite a character herself, stereotypically pessimistic with the fury of a berserkerkang, just far more intelligence coupled with the battle rage.
For an idea of their characters:
"I've always said you can get more with a two - by - four and a kind word than with just a kind word." - Marcus Cole.
"I would like you to take time to learn the B5 Mantra:'Ivanova is always right. I will listen to Ivanova. I will not ignore Ivanova's recommendations. Ivanova is God.' Have a nice day." - Susan Ivanova.
As you can see they're cracked. ^_^
At the close of Season 4... they are on the white star fleet (ships specifically built for the Rangers). Susan is commanding and marcus is there as translator since most of the crew only speaks Mimbari, and her self-taught Mimbari ... is atrocious). The ship receives battle damage... debris comes crashing down, and Susan is mortally wounded... being shipped back to Babylon 5 to die.
Marcus just can't handle it... and he goes through every file in MedLab to try to make a miracle for her, and finds it, an alien machine that takes one person's life-energy and transfers it to another. It had been used as penal punishment... criminals would be sentenced to give up the amount of life according to the crime.
Marcus hooks himself up, knocks at the on crew staff in MedLab, and gives his life to Susan, saying what he could never say to her at any other time, that he loved her.
Later Susan loses it, everyone she has ever loved has died or betrayed her. She rants to the doctor Stephen that she thought she heard God saying "I love you" and he even had an English accent like in all the old media...
It breaks her in a way, the rest of her life is devoted to duty. She can't stand the station with so many memories and leaves, to Cpatain her own ship, the Valkyrie. The creator, JMS, never does anything without a damn good reason... and considering that she has lost all she has loved... she becomes one who chooses out the battle slain.
Towards the end, after she retires from the Earth military... she becomes "The One." Marcus had lived for the One, and died for the One... he was just a few decades early.