On Buffy's 18th birthday, Giles must test her abilities. He drugs her so she'll lose some of her strength and she is sent to face a powerful vampire. When the head of the council delivers the results, Giles is fired as its felt that his fatherly feelings for Buffy make him an impartial and inadequate watcher.
Even though there is a new watcher in charge, Buffy still turns to Giles for advice, which causes tension between everyone.
When the Mayor ascends, Giles is given the important task of actually killing him. Buffy traps the new snake mayor and Giles blows him, and the school up.

With Buffy in college and the school blown up, Giles is at somewhat of a loose end. Tempers are frayed at Thanksgiving when Buffy decides that she will cook at his house and a Chumash indian is running amock. No-one can decide the best course of action to take.
Feeling neglected and pushed out of everyone's lives, Giles meets up with Ethan and ends up getting drunk with him. Ethan casts a spell on him and he turns into a Fyarl demon. With Spike's help, he tries to find a cure while an unsuspecting Buffy catches up with him and stabs him. Thankfully she doesn't kill him and realises that the demon is actually Giles.
Just before things come to a head with Adam, Giles expresses his feelings about being pushed out of the loop, primarily by getting drunk and leaving Buffy to it. They manage to slove their differences and combine to defeat Adam.

In an effort ot quell his boredom, Giles buys the Magic Box. Within hours of its opening, a demon hunting for Buffy bursts in and knocks him out.
In order to get away for a while, Giles returns to England. His activities there remain unknown.
Shortly after his return, Joyce dies, throwing everything into chaos. Buffy refuses to listen to reason and emotions are high amongst the scooby gang.
With Glory on the hunt for Dawn, the gang go on the run. Giles is shot and, stuck in the middle of nowhere, things look bleak for him until Ben arrives and delivers medical attention to his injuries.
In the final fight against Glory, Giles is left with no choice but to kill her while she is in the guise of Ben, in order to stop her ever returning to get payback on Buffy.

After Buffy's death, Giles returns to England once again, feeling that he has little use in Sunnydale. Although his stay is brief as he returns when she is resurrected.
Giles feels he is standing in Buffy's way and, yet again returns home to England. It is only when a powerful coven tell him of a dark force rising in Sunnydale, that they bestow him with the 'true essence of magic' and he goes back to Sunnydale to face Willow. He tries reaching out to her with no success, his efforts to contain her with a binding spell are short lived and she beats him to a bloody pulp before stealing the magicks he has.
These magicks tap into the last bit of humanity left in her and allowed Xander to reach her before she destroys the world.