Anyway, you there! Thanks for feeling bad about Buffy. Have a drink. Okay, so how did she die? Well, it's like this. There's some kind of organization of demon types that's trying to bring about a state of total chaos. Right? Okay. So, they need Buffy's little sister Dawn in order to do this. Dawn's this "key" person or something; and she's totally innocent and hasn't a clue about this demon stuff. No, wait, she does have a clue. But she's not a demon. She's really sweet and all that. Or something.
Anyway, this bad demon who's a god comes along and takes Dawn away to sacrifice her so this world-ending thing can happen. Okay? Okay. So, Buffy and her pals have to rescue Dawn and also prevent the end of the world. So how come they can't just call the police? Beats me too. Anyway. . . .
Willow and Spike and Anya and Xander and Giles and this other gal whose name I can't remember all help Buffy by beating up bad guys and such. Buffy climbs this great big tower where Dawn is tied up, but it's too late and the end-of-the-world thing is starting! Yeah. So Dawn says she's got to jump off this big tower to stop this ritual from ending the world.
Buffy says, "Oh no, that's my job little sis." Or words to that effect. And so Buffy jumps off the tower into this floating ball of something-or-other and is killed, but the end-of-the-world thing stops and the world is saved. But Buffy's dead and they show a picture of her grave.
Okay, that's what happened more or less. Have a drink.