K buddy, see it's set in the 2010s in a world where, like, the British Empire still exists, or Britannian empire, whatever you wanna call it, yo.
Right, so this Lelouch dude, he's like some ordinary student kid (or so you think at first, dude). Yeah, and he's got like, his best buds, and they all live normal lives in a fancy school and shit.
BAM! He gets this powah called Geass, right? He can control peoples minds with that shit, it's awesome. So he decides that he'd defeat the Britannian Empire because they took Japan, 'cos I guess he likes Japan or some shit, I dun' remember. Also his mother is dead, so he wants to save her, or something, can't remember that either. So he becomes this Zero guy and wears this fucking helmet mask shit.
Now at the same time, his childhood friend, Suzaku (call him Suzy, right) is all BAM BAM BAM with the Britannian Empire, so Suzy and Lelouch end up having to, like, you know, fight each other, even though they were buds.
Shit happens, people die, yo. Like that pink haired chick, but that's a different matter, right? Anyway, it turns out Lelouch is actually the son of the fuckin' Britannian Emporer, and Suzy is the son of the previous Japan Prime Minister and Szy fucking KILLED him.
Right, so the ending was all like, Suzy corners this Zero dude, finally, and shoots off his mask, and he's proper shocked at 'cos it's Lelouch and all. And then they put their guns up and are like LEELOUUUUUUCH, SUZYYYYYYYY and shit. And then one of 'em shoots the other but you know what, THEY DIDN'T SHOW WHO MAN
Anyway yeah, that's the story o' the Geass. Remember it, bro.