After attempting to start a new life in Los Angeles, Buffy Summers (Sarah Michelle Gellar) returns to Sunnydale in season three. Angel (David Boreanaz) is resurrected due to unknown circumstances. While Buffy is happy to have Angel back, he seems to have lost much of his sanity in Hell. Buffy helps Angel recover but due to Angel's role as Angelus, Buffy's friends don't trust him.

     Rupert Giles (Anthony Stewart Head) is fired from the Watchers' Council because he has developed a "father's love" for Buffy, and towards the end of the season Buffy announces that she will also no longer work for the Council. Early in the season she is confronted with an unstable Slayer, Faith (Eliza Dushku), who was called after Kendra's death near the end of season two.

     Angel after gets his soul back, he is once again haunted by his guilt personally by an entity called the First Evil, who tries to get Angel to commit suicide. Buffy prevents Angel from killing himself but he still feels heavy remorse for his past actions.

     Although the First is still out there, the antagonist of the season is shown to be, the affable Sunnydale Mayor Richard Wilkins III (Harry Groener), who has plans to become an Olvikan on Sunnydale High's Graduation Day. A goal he had in mind after gaining immortality after a fustian bargain with demons when he originally founded Sunnydale.

     Although Faith works with Buffy at first, after accidentally killing a human, Faith becomes irrational and sides with Mayor Wilkins, whose charismatic behavior influences Faith's dark side. She even starts to help Wilkins in his plan, although she falls into a coma after a fight with Buffy.

     At the climax of the season, Wilkins speaks at the graduation ceremony, as the time for his transformation has come and he finally morphs into Olvikan. He kills several people, including Principal Snyder. Buffy and her friends get the graduating students to fight back against Wilkins. Buffy confronts the demon, taunting him about Faith. She lures the provoked Mayor into the library which was rigged with dynamite. The explosion is set off, destroying Wilkins and his Olvikan form.

(From Wiki)