When Rupert Giles came to America, he came with a purpose: to stop the killing. After doing extensive research on the subject, he learned that the Hellmouth, a site of mystical convergence existed in Sunnydale, California.
Being a wealthy man, he bought himself the nearest mansion and set up shop, preparing for the day he would approach a Ms. Buffy Summers.
He worked alongside the Initiative [Maggie and Adam Walsh], a duo known for their pretentious need to identify themselves as some anonymous entity, and came up with a research facility, underground. His wife, the American Jennifer Calendar, helped establish a computer system which filed all the evils that they were to fight.
When all was prepared, the mansion - which is haunted by a spirit named Dennis - welcomed several new arrivals:
Xander Harris - Giles' ward, who Jenny took a quick shining to.
Anya Jenkins - The former vengeance demon who kept house, as well as being an authority on demon languages.
Cordelia Chase - A woman with powerful visions, capable of predicting the nearest danger, and with a latent telekinetic ability.
Tara Maclay - An eager research assistant.
Riley Finn - A former military man, with a wandering (into closets) spirit.
Joyce Summers - A caring mom who objected to her daughter being in a superhero "gang." She was sent to live inside the wall of the mansion, to "keep watch."
And the four champions:
Buffy Summers - The Slayer.
Spike T. Bloody - The vampire with a chip in his head; incapable of causing harm to anyone but demons.
Willow Rosenberg - A powerful witch.
Oz Osbourne - A werewolf, who learned to control his changing through tantric sexual practices.
And they are here to fight said evil.
And evil... won't win, probably.