Warning: Wikipedia contains spoilers.

Table of contents
1 Season One
2 Season Two
3 Season Three
4 Season Four
5 Season Five
6 Season Six
7 Season Seven

Season One

Buffy the Vampire Slayer was a mid-season replacement series, including only 12 episodes in its first season. The first season begins several months after the events of the movie, with Buffy Summers, already knowing she is the Slayer, arriving in Sunnydale, along with her somewhat oblivious mother, who believes her daughter is simply a troubled teen-ager. Buffy soon meets her Watcher, the librarian Rupert Giles, and the friends who will become her demon-fighting comrades, and they discover that the High School is located on a "Hellmouth," a portal which serves as a nexus and attracting point for a wide variety of supernatural beings and occurrences. Among other things, the Hellmouth causes many of the fears and trials of the school's adolescents to manifest as supernatural phenomenon. As a basic example, a girl who feels neglected may find she has literally become invisible. This kind of metaphor is frequently employed throughout the series.

The first season includes mostly stand-alone episodes focusing on a particular supernatural phenomenon. The overarching plot of the season follows "The Master," an ancient and very powerful vampire who is trapped in the entrance to the Hellmouth and looking for a way to get free (most likely, by opening the Hellmouth and setting loose all of the horrors which lie beyond). They typically employ a combination of detective work, emotional exploration, and frequent physical combat to solve these problems, guided extensively by the research of ancient mystical texts. The gang is also aided by a mysterious stranger named "Angel," who Buffy soon falls in love with, before discovering that he is in fact a vampire who suffers from a Gypsy curse which restored his human soul, making him basically "good" but also tormented by guilt.

Ultimately, the Master sets his plan in motion, and is released from his prison, killing Buffy in the process. She is quickly revived, however, the Hellmouth is closed, and Buffy kills the ancient vampire before he can do any harm to the world.

Season Two

Season two continues with many stand alone, supernatural-high-school focused episodes. The primary action of the season, however, revolves around the tragic relationship between Buffy and her vampire lover Angel. The Master is replaced by the younger vampire couple Spike and Drusilla. Halfway through the season, Buffy and Angel finally make love for the first time; however, due to an unknown caveat in his gypsy curse, this results in Angel losing his soul and becoming evil. This is perhaps the most potent example of the show's metaphorical mandate to explore universal adolescent themes: Angel turning evil as an allegory for the girl who sleeps with a man and discovers that he now won't return her phone calls. Angel joins with Spike and Drusilla, and they go about the business of tormenting and trying to kill Buffy and her friends.

Season two introduces several other new characters, including Oz (a bass player and werewolf who becomes Willow's lover), Kendra (another Slayer, "called" during the brief moments that Buffy was dead), Jenny Calendar (a descendant of the Gypsies who cursed Angel), and Ethan Rayne (a somewhat inept evil warlock who ran with Giles during his rebellious youth).

Ultimately, Angel discovers an ancient artifact which he can use to destroy the world (specifically by sucking every living thing into hell). The particulars of the ritual dictate that, once Angel opens the portal, the only way to close it is by killing him. The gang hopes to either kill Angel or restore his soul before he can perform the ritual: however, in a classically tragic conclusion, they succeed in restoring his soul only after he has performed the ritual. Even though Buffy's love has been restored and made "good" again, she still must kill him to save the world. In the end, having been expelled from school, revealed her demon-fighting life to her mother, and killed her true love, Buffy leaves Sunnydale with the intention of never coming back.

Season Three

Buffy returns to Sunnydale after many months in hiding. Soon thereafter, Angel also returns from hell (actually an alternate dimension where time runs much faster; his time of torment has therefore been much longer than at first apparent). The first two seasons included several hints that the unseen Mayor of Sunnydale might have some hand in the town's evil happenings. The evil Mayor (Richard Wilkins) emerges as the major villain in the third season. (As a side note, the series writers often plotted the show to conceal the true identity of the primary villains until well into the season. Once discovered, seasonal villains are known affectionately by fans as "The Big Bad.")

The story is complicated by the arrival of another Slayer, Faith, who was called after Kendra was killed at the end of season two. Faith is a leather clad "bad-girl" with a severely abused childhood, which causes her to be very unstable; Faith takes particular pleasure in violence, and after accidentally killing a human being (strictly forbidden for a Vampire Slayer) she ultimately turns rogue and joins with the evil mayor. The Mayor, who has intentionally over a century built the town of Sunnydale "for demons to feed on", now has plans to "ascend" from human into pure demon form. Pure demons, we discover, are much bigger and more destructive than the impure demons Buffy and her comrades usually fight. The major action of the season revolves around trying to discover the details of his plan, and trying to deal with the increasingly dangerous (though poignantly conflicted) Faith.

The third season also sees the introduction of Wesley Windham Pryce, another Watcher sent to replace Giles (who the Council of Watchers feels has grown too emotionally attached to Buffy). Though Wesley was generally treated as inept and mostly played as a comic fop on Buffy, the character went on to become a more heroic figure as a regular on the spin-off series Angel. Season three also saw the expansion of the minor recurring characters Jonathan, who would become a major player later in the series, and the former vengeance demon Anya, who would later become a series regular.

The season ends with Buffy having to stab Faith, putting her in a "permanent" coma. The Mayor does ascend into demon form at Sunnydale High School graduation, and to kill him, the gang blows up the school with him in it, simultaneously killing the evil creature and fulfilling the dream of many disgruntled students throughout history.

Also in the final episode, believing that his life with Buffy can only be destructive to her, Angel leaves Sunnydale (to live in L.A. and to star in his own spin-off series). (Though no mention was made of it at the time, regular character Cordelia Chase made the same move.)

Season Four

Season four begins with Buffy and Willow enrolling at Sunnydale University, whilst Xander begins his working life. As the first three seasons dealt metaphorically with the struggles of high school students, in season four the series creators set out to do the same with college (and non-college post-high school life), dealing with issues of leaving home, redefining family relationships and friendships, and striving to adapt to more complex responsibilities and social pressures.

Though Sunnydale U does not rest on a Hellmouth, it soon is revealed to sit above a top-secret military installation, called the "Initiative," which is manned primarily by secretly recruited fraternity brothers. Though it appears at first to be a well-meaning demon fighting operation, it eventually becomes clear that the Initiative is actually designed to perform secret tests on supernatural beings, presumably in a clandestine effort to develop military weapons. Among other things, the Initiative has combined demons, humans, and cybernetics into a prototype super-creature, affectionately named "Adam". Adam escapes the Initiative and masterminds a plan which would ultimately see all humans either destroyed or turned into demon/cyborg creatures like him.

Though Buffy and her friends grow more and more estranged throughout the season, they eventually reunite and defeat Adam through the use of a spell which combines all of their powers into Buffy's body. Thus endowed, Buffy kills the demon/cyborg, and the entire Initiative is destroyed in the process.

Season four includes a troubled romance between Buffy and college student/Initiative soldier Riley. It also details Willow's increasing interest in the Wiccan arts and, perhaps more noteworthy, an ongoing romantic affair with fellow Wiccan student Tara; this homosexual relationship was somewhat controversial, though the show's creators were determined to treat it in a respectful and unsensational manner. Season four also saw the re-emergence of the vampire Spike as a regular character, who is "neutered" by the Initiative by implanting a microchip in his brain which prevents him from being able to do any physical harm to human beings. This "chip" allowed Spike to assume a place as one of the series' most vibrant and morally ambiguous characters.

Season Five

The fifth season saw the surprise introduction of Buffy's younger sister Dawn, who previously never existed. Though Dawn's origins are at first kept mysterious, it is eventually revealed that the fourteen-year-old is actually a mystical Key, which was once pure energy but has now been given human form as The Slayer's sister. This transformation was made by a group of now-extinct monks, who created the girl from the Key (using Buffy's essence) and gave Buffy and all those around her a lifetime worth of completely convincing false memories. All of this was done to protect the Key from the evil Glory, a powerful exiled God who, among other things, shares her body with a human man and feeds off the mental energy of humans.

The primary action of season five revolves around Glory's efforts to locate the Key, and Buffy and her friend's struggle to discover Glory's true plans and to protect the innocent Dawn. The season also details Dawn's struggle to accept what she is, as well as Spike's ongoing unrequited love for Buffy. A dramatic highlight of the fifth season was also the unexpected death of Buffy's mother (from completely natural causes) in an episode which won the series its widest critical acclaim.

Glory eventually discovers the Key's new form, and determines to sacrifice the girl in a ritual which would bring down the walls that separate different dimensions, resulting in absolute chaos and torment for every living thing. In a similar conflict to the one in season two, it is revealed that once the ritual is performed, the only way to save the world would be to kill Dawn. However, once the ritual has begun, Buffy realizes that she and Dawn share the same essence, and rather than let her sister die, Buffy throws herself into the vortex and sacrifices her own life. Season five ends with the world safe, and Buffy dead and buried.

Season Six

Season six begins with Buffy being resurrected through the use of a powerful spell. In the mythology of Buffy, this is possible because her death was not "natural". Though her friends assume that she, like Angel, must have been dead in a tormented Hell dimension, Buffy slowly reveals to them that she was in fact in a blissful heavenly world; the season deals largely with her sorrow at being brought back and having to continue the fight, thus moving the show's metaphorical focus from the trials of adolescence to the struggles of early adulthood. Buffy struggles with loneliness, earning money at a degrading job, and being the forced matriarch of her "family".

Throughout season six, the arch villains seem to be "The Troika", a trio of comical, though genuinely dangerous, geeks from Sunnydale High School who have joined to use their nerdy genius to take over the town. They are Jonathan, who featured in earlier seasons mostly in the role of victim, Warren, who appeared in season five as the builder of a life-like robot girlfriend, and Andrew, whose brother Tucker had appeared in season three attempting to massacre the students at the Prom.

A persistent subplot in the season involves Willow and her growing addiction to destructive dark magics; this sub-plot becomes the climax of the season when Willow's lover Tara is killed by one of The Trio, and Willow descends completely into darkness and goes on a destructive rampage, at first to take revenge, and ultimately to relieve her own suffering by destroying the entire world. In the end, the unconditional love and forgiveness of her friend Xander brings her back from the edge, saving the world in the process.

Season six also continued the story of Spike's unrequited love for Buffy (they have a somewhat violent and ambivalent sexual affair throughout) ending with Spike attempting to rape her and leaving Sunnydale, seemingly in search of a tool of vengeance, but ultimately to have his own human soul restored to him. This season also saw the departure of Rupert Giles as a regular character (he returns frequently as a special guest).

Season six was the first of two seasons in which Buffy aired not on the Warner Brothers television network but on the United Paramount Network (UPN).

Season Seven

Buffy's final season revolves around the mysterious "First Evil," a demonic force which, among other things, can appear in the form of any person who has died. "The First", as it became known, had previously appeared in season three attempting to convince Angel to commit suicide by sunlight.

The season also involves a number of young girls who are "potential" Slayers, gathered together by the now destroyed Council of Watchers, and the ongoing spiritual struggles of Spike, who now lives with the remorse of a soul. Much of the story takes place at the newly reconstructed Sunnydale High School, bringing the series and its characters back to their roots, The Hellmouth.

As the season progresses, it is revealed that the First Evil has been gathering an army of "Pure" vampires, much more powerful and violent than the normal humanesque variety. It is never made entirely clear what the First Evil hopes to gain by releasing this army on the world.

In the end, there is a climactic battle within the Hellmouth itself, during which Willow invokes a magical spell which results in every girl in the world with the potential to become a Slayer being "activated". With her small army of girls now endowed with full Slayer power, they manage to hold back the army of vampires long enough for a powerful Amulet worn by Spike to take effect: though this Amulet vaporizes all of the creatures in the Hellmouth, it also kills Spike in the process, and sparks an earthquake which destroys the entire town of Sunnydale, leaving nothing but an empty crater where the town and the hellmouth used to be. The show ends its run with the Hellmouth closed, and Buffy no longer the only "Chosen One."