
Buffyverse is enormous; here are the relevant bits.
Overall:
- magic and monsters are real, but most people don't notice
- vampires are demons possessing dead bodies; vampire + human conscience only in exceptional circumstances
- a new Vampire Slayer gets powers when the last one dies; always a young woman; has super-strength, etc
- Watchers train the Vampire Slayers and are almost uniformly Old Rich White Dudes with bad ideas
- Giles is one of these but marginally less out-of-touch than most
Rupert Giles: (called "Giles" by literally everyone)
- Age 10, Giles shipped off to Watcher's Academy. Becomes uptight head of class. Witnesses slaughter of friends in final exam gone wrong.
- Giles says OK FUCK THAT, runs off to London, joins a punk band, does magic drugs with new bro Ethan.
- They + friends tattoo a demon's mark on themselves, get high on possession, it Ends Poorly. Giles + Ethan kill friend accidentally.
- Giles' cool grandma drags his depressed punk ass back home.
- MUCH LATER in S1: Giles goes to California, where he is Very British and Uncool but becomes everyone's dad. Trains Buffy + friends to fight monsters.
- S2: Buffy's vampire boyfriend loses his soul, kills Giles' girlfriend, tortures Giles. (It takes Giles 6 seasons and literal death to be friends with him again.)
- S3: Giles betrays Buffy as is protocol, then comes to her rescue at the last minute. Gets fired from Watchers.
- S4: Buffy's other vampire boyfriend lives chained up in Giles' bathtub for a while.
- S5: Owns a witchcraft supply shop. Buffy gets a teen sister who retcons herself into their memories.
- S6: Buffy dies, comes back depressed, Giles leaves and thinks he's helping. Willow turns evil; Giles takes her to England for rehab.
- S7: Adopts a bunch more potential Slayers, plays D&D during the apocalypse. Sunnydale destroyed.
- S8: Becomes bros with bad-girl-Slayer Faith; Buffy has more ill-advised vampire boyfriend sex and the world starts ending. Giles dies.
- S9: Rescued from demon hell by vampire boyfriend. Gets resurrected as a twelve-year-old with adult memories.
- S10: everyone knows about magic now and Giles can't handle his hormones
the end.