At the beginning of season six Willow resurrects Buffy with a powerful spell. Although her friends believe that they have taken her out of Hell, Buffy had actually been in Heaven since her death (a fact that is revealed to her friends later in the season). She is therefore traumatized by her resurrection and this causes her to fall into a deep depression for most of the season.
Giles returns to England after realizing that Buffy has become too reliant on him, while Buffy takes up a fast-food job for money and develops a secret, mutually abusive relationship with Spike.
Dawn is happy to have her sister back, but Buffy's depression causes Dawn's feelings of alienation to lead to kleptomania. When demon briefly traps everyone in the Summers' house, Dawn's feelings are revealed and Buffy promises to help mend their relationship and spend more time together.
On the wedding day of Xander and Anya, a magician, seeking his own vengeance against Anya, tricks Xander into calling off the wedding by showing him a future where they are both miserable. Anya reverts back to a vengeance demon once more. When Anya can't find anyone who wishes vengeance on Xander, she has a one night stand with Spike, endangering Spike's relationship with Buffy.
Willow becomes addicted to magic. Tara becomes concerned and challenges Willow to go a week without magic, but Willow gives in to her addiction, causing Tara to leave her. Despite their separation, Tara is still devoted to helping Willow with her addiction, leading to their reconciliation.
They also begin to deal with The Trio, a group of nerds led by Warren Mears who use their technological and magical proficiency to attempt to kill or neutralize Buffy and take over Sunnydale, a goal they had in mind since the beginning of the season. The Trio would often use monsters such as vampires and demons to assist them.
Warren is shown to be the only competent villain of the group, and his capacity for evil is demonstrated by the attempted rape of his ex-girlfriend, which leads to her death. After Buffy thwarts his plans multiple times and the Trio breaks apart, Warren then attacks Buffy with a gun but accidentally kills Tara with a stray bullet. Willow tries to resurrect Tara but fails.
Tara's death causes Willow to descend into darkness and unleash all of her dark magical powers. Willow hunts down and flays Warren alive in vengeance. Driven by grief, Willow fights and overpowers Buffy, who is forced to fight when she is unable to condone Willow's actions. Giles returns to help face Willow in battle and infuses her with light magic, tapping into her remaining humanity. This backfires when Willow feels pain in everyone around the world and decides to destroy the world in order to end everyone's suffering. At the climax of the season, Xander prevents Willow from destroying the world by continually telling her he loves her (as a friend). She eventually breaks down, and the dark magic drains from her. Meanwhile, Buffy comes to terms with being alive again, and promises to be there for her friends.
At the end of the season, after attacking Buffy, Spike leaves Sunnydale and travels to Africa to see a demon and asks him to "make him to what he used to be" so that he can "give Buffy what she deserves." The audience is led to believe that he was referring to the freedom to be evil (unrestrained by his punishment chip) and thus able to kill Buffy; however, after passing a series of tests, Spike is rewarded with the restoration of his soul. This raises issues two years later, when he joins Angel's party and Angel has to deal with being no longer the only "vampire with a soul."