In every generation there's a chosen one.

A short schematic take on one of the best shows on TV. Based on my Two Trilogies Theory.

Occurrence The First Trilogy (seasons 1-3): The years of the mother (Joyce) and the first vampiric boyfriend (Angel). It’s the high school years. The years of growing up, maturing and accepting her role in life. The bridging season (season 4). The year of change. The year between maturing and becoming mature. Realizing she has just begun. The Second Trilogy (seasons 5-7): The years of the sister (Dawn, formally a Key, that becomes a surrogate daughter of sorts) and the second vampiric boyfriend (Spike). It’s the years of being an adult. Taking care of things and people. The pain of making mistakes. Comment
Season 1 Season 5
There is a vampire in Sunnydale that is inhibited in his usual ways. Angel had been cursed by Gypsies. He got his soul back and that has been an important factor in turning him towards de side of good. Spike had been captured the previous season by a covert scientific and military operation called ‘The Initiative’ and was given a neural chip in his brain that stops him from harming people. It sets him on a path towards the side of good. The main difference being the origin of the inhibitor of course. In the case of Angel it was more ‘natural’, but in both cases they did not ask for it.
The very same vampire falls in love with the Slayer (Buffy Anne Summers). Angel practically worships her and is inspired by her. Before he met her he was in a very sorry state and certainly not completely fit to fight the good fight. Spike, to his dismay, wakes up from a nightmare and discovers that he has fallen in love with Buffy. He fights it, but in the end he has to give in to his feelings. Buffy herself becomes very much in love with Angel, but at this stage is rather disgusted with Spike and his feelings for her.
In the final episode of the first season of the trilogy Buffy faces the Big Bad of the season, and during that episode she dies. Buffy confronts The Master and he drowns her in a pool of water (‘Prophecy Girl’). Buffy sees no other way than sacrificing herself. Only through her death she can close the dimensional portals that have been opened and are threatening to destroy Earth. In ‘The Gift’. In the case of the drowning Xander Harris gives her the kiss of life of course. Something Angel could never do. At the end of season five she stays dead until the beginning of the sixth season.
Season 2 Season 6
At the beginning of the second season of the trilogy she acts strange and does not feel all that well. The season opener ‘When she was bad’. It only really lasts one episode. During the first half of the season she fears she hasn’t come back completely right. The fact that Spike can hurt her despite his chip seems to confirm it. The first time around it was only one episode. The second time around lasted longer, but Tara McClay assured her that there was nothing wrong with Buffy.
Buffy makes love with her vampire. Angel and Buffy go all the way in ‘Surprise’. In ‘Innocence’ she discovers something horrible has gone wrong. Angelus emerges and goes very much to the Dark Side. Buffy enters into a destructive sexual relationship with Spike and is convinced something is already very wrong. Angel of course turned back into Angelus due to the ‘perfect happiness’ clause. Spike had no such problems. He just had a chip. It does further change him!
Someone very close and dear to Buffy becomes her enemy and wants to kill her and the whole world. Angelus wants to destroy Buffy and wants to use the demon Acathla to suck in planet Earth into a Hell dimension. Willow, grief stricken after the death of Tara is set to destroy the entire world to end all suffering. Buffy is able to close Acathla by sending a bleeding Angel into his vortex. Willow is stopped by the words of her best friend Xander.
Season 3 Season 7
The destruction of Sunnydale High School. Sunnydale High gets blown up. Sunnydale High gets rebuild. But is destroyed when the entire town of Sunnydale goes.
Buffy leads a large army against the big bad of the season. The Mayor turning demon faced a large body of Sunnydale students helping Buffy saving them. Adam’s troops get decimated by the combined might of the Scoobies and the remains of The Initiative military. The First’s Turoc Han army is destroyed when Buffy leads her Scoobies and an army of Slayers into the Hellmouth. Massive fighting. Many participants. Great mayhem.
At the end of the season her vampire lover goes away. Angel leaves and goes to Los Angelus. Spike sacrifices himself. Of course he is resurrected again on ‘Angel’. In Los Angelus.