Season one exemplifies the "high school as hell" concept. Buffy Summers has just moved to Sunnydale after burning down her old school's gym and hopes to escape her Slayer duties. Her plans are complicated by Rupert Giles, her new Watcher, who reminds her of the inescapable presence of evil. Sunnydale High is built atop a Hellmouth, a portal to demon dimensions that attracts supernatural phenomena to the area. Buffy meets two schoolmates, Xander Harris and Willow Rosenberg, who help her fight evil through the series, but they must first prevent The Master, an ancient and especially threatening vampire, from opening the Hellmouth and taking over Sunnydale.The emotional stakes are raised in season two. New vampires Spike and Drusilla (weakened from a mob in Prague, which presumably caused her debilitating injury), come to town along with the new slayer, Kendra Young, who was activated as a result of Buffy's brief death in the season one finale. Xander becomes involved with Cordelia, while Willow becomes involved with witchcraft and Daniel "Oz" Osbourne, who soon becomes a werewolf after being bitten by a young cousin who just happens to be a werewolf. Buffy and the vampire Angel develop a relationship over the course of the season, but after they sleep together, Angel's soul, given to him by a curse, is lost and he once more becomes Angelus, a sadistic killer. He torments much of the "Scooby Gang" throughout the rest of the season and murders multiple innocents and Giles' new girlfriend Jenny Calendar, a gypsy who had been sent to make sure that the curse that gave Angel his soul was never broken. Buffy is forced to kill him (right after Willow restores his soul) and leaves Sunnydale, emotionally shattered.
After attempting to start a new life in Los Angeles, Buffy returns to town in season three. Angel is resurrected, but after he and Buffy realize that a relationship between them can never happen, he leaves Sunnydale at the end of the season. Giles is fired from the Watcher's Council because he had developed a "father's love" for Buffy, and towards the end of the season Buffy announces that she will also no longer be working for the council. Early in the season she is confronted with an unstable Slayer, Faith, who was called up after Kendra's death near the end of season two, as well as affable Sunnydale Mayor Richard Wilkins, who has plans to "ascend" (become a giant snake demon) on Sunnydale High's Graduation Day. Although she works with Buffy at first, after accidentally killing a human, Faith becomes irrational and sides with Mayor Wilkins, eventually landing in a coma after a fight with Buffy. At the end of the season, Buffy and the entire graduation class defeat Mayor Wilkins by blowing up Sunnydale High, killing him in the process.Season four sees Buffy and Willow enroll at UC Sunnydale while Xander joins the workforce and begins dating Anya, a former vengeance demon. Spike returns as a series regular and is abducted by The Initiative, a top-secret military installation based beneath the UC Sunnydale campus. They implant a microchip in his head which prevents him from harming humans. He reluctantly helps the Scooby Gang throughout the season and eventually begins to fight on their side after learning that he can harm other demons. Oz leaves town after realizing that he is too dangerous as a werewolf, and Willow falls in love with Tara Maclay, another witch. Buffy begins dating Riley Finn, a grad student whom she later realizes is a member of The Initiative. Although appearing to be a well-meaning anti-demon operation, The Initiative's sinister plans are revealed when Adam, a demon/human/computer hybrid secret project, escapes and begins to wreak havoc on the town.
During season five, a younger sister to Buffy, Dawn, suddenly appears in Buffy's life, and although she is new to the series, to the characters it is as if she has always been there. Buffy is confronted with Glory, an exiled hell-God that is searching for a "Key" that will allow her to return to her Hell dimension and in the process would blur the lines between dimensions and unleash Hell on Earth. It is later discovered that the Key's protectors had turned the Key into human form as Buffy's sister Dawn, concurrently implanting everybody with lifelong memories of her. The Watcher's Council aids in Buffy's research of Glory, and she and Giles are both reinstated by the Council. Riley leaves early in the season after deducing that Buffy does not love him and joins a military demon-hunting operation, while Spike, still implanted with the Initiative chip, realizes he is in love with Buffy and continually helps the Scoobies in their fight. Buffy's mother, Joyce, dies of a brain aneurysm, while at the end of the season, Xander proposes to Anya. Glory later discovers that Dawn is the key and kidnaps her. Buffy sacrifices her own life to save Dawn's and prevent the portal to the Hell dimensions from opening.
At the beginning of season six, Buffy's friends resurrect her through a powerful spell, believing that they have rescued her from Hell. Buffy reveals she was in Heaven during her death and she falls into a deep depression for most of the season. Giles returns to England after deciding 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 suffers from kleptomania and feelings of alienation, Xander leaves Anya at the altar, after which Anya once again becomes a vengeance demon, and Willow becomes addicted to magic, causing Tara to temporarily leave her. They also begin to deal with The Trio, a group of nerds led by Warren Mears who use their technological proficiency to attempt to kill Buffy and take over Sunnydale. Warren is shown to be the only competent villain of the group and, after Buffy thwarts his plans multiple times and the Trio breaks apart, he comes unhinged and attacks Buffy with a gun, killing Tara in the process. This causes Willow to descend into darkness and unleash all of her dark magical powers, killing Warren. Giles returns to face her in battle and infuses her with light magic, tapping into her remaining humanity. This causes Willow to attempt to destroy the world to end everyone's suffering, although it eventually allows Xander to reach through her pain and end her rampage. At the end of the season, after attacking Buffy, Spike leaves Sunnydale and travels to see a demon and asks him to "return him to what he used to be" so that he can "give Buffy what she deserves". After passing a series of tests, the demon restores his soul.
During season seven, it is revealed that Buffy's resurrection caused an instability which allows the First Evil to begin tipping the balance between good and evil. It begins hunting down and killing the inactive Potential Slayers, and raises an army of ancient, powerful Turok-Han vampires. After the Watchers' Council is destroyed, a number of the Potential Slayers (some brought by Giles) take refuge in Buffy's house. Faith returns to help fight the First Evil, and the new Sunnydale High School's principal, Robin Wood, also joins the cause. The Turok-Han vampires and a sinister preacher known as Caleb begin causing havoc for the Scoobies. As the Hellmouth becomes more active, nearly all humans and demons flee Sunnydale. In the series finale, the Scoobies descend into the Hellmouth while Willow casts a spell that activates all of the Potential Slayers, granting them Slayer powers. Angel comes to Sunnydale with an amulet, which Buffy gives to Spike. Anya, now human again, dies in the fight, as do some of the new Slayers. Spike's amulet channels the power of the sun and kills all of the vampires in the Hellmouth, incinerating Spike in the process. This causes the Hellmouth to collapse, and the entirety of Sunnydale collapses into the resulting crater, while the survivors of the battle escape in a school bus.