With examples from Buffy the Vampire Slayer and usage guidelines from Garner’s Modern American Usage.
The questioner gets it in one! Well done, callistosh65. From a definition standpoint, there is no difference between “toward” and “towards.” The only real distinction is that American English users tend to go with “toward” while British English favors “towards.”
That is not to say either usage is exclusive geographically. There is enough usage overlap that one or the other is unlikely to throw a reader out of the flow of the story. But yes, careful fanfic writers will add that terminal “s” for their British fandoms and leave it off for the U.S.
And don’t forget character dialogue within the fandom. In a Buffy the Vampire Slayer fic, you might talk about Buffy moving toward the mausoleum door, and when the California natives speak, it’s like this:
Willow: Do you guys have a gig tonight?
Oz: Oh, no. Practice. See, our band's kinda moving toward this new sound where ... we suck, so, practice. (BTVS, Surprise)
But the British English speakers would be written this way:
Willow: By the way, are we hoping to find a body, or no body?
Xander: Call me an optimist, but I'm hoping to find a fortune in gold doubloons.
Giles: Um, body would mean flesh-eating demon, no body would point towards the, ah, army of zombies thing. Take your pick, really. (BTVS, Some Assembly Required)
Just a bonus little usage note here, “toward/towards” is sometimes used where “to” or “against” would be more appropriate. Remember, “toward/towards” implies movement. Therefore, “to” would be a better choice here:
“Sacred duty, chosen one, obligation toward all humanity, blah, blah, blah. I know the drill, Giles.”
Phrases like “prejudice toward” and “a grudge toward” would be better served by “against.” This provides a sense of opposition rather than simply “in the direction of.”
“It’s not like I’m prejudiced toward poor people,” Cordelia said, “I just don’t see why they have to subject decent society to their Blue Light Specials. Did you see that sweater?”
The good news is that the usage is flexible enough that you can follow your preferences without being wrong. The better news is that you can wield the apparently utilitarian and unglamorous "toward/towards" to achieve subtleties that are a delight to lovers of the language.
As Xander said in Band Candy, "There is no bad here."