While chasing a vampire in a cemetery, Buffy meets Dracula, who has come to Sunntdale to meet her. Buffy feels proud with the revelation of the greatest vampire in the world. Meanwhile, Giles secretly tells Willow that he will return to England since Buffy does not need his service of watcher anymore. Dracula turns Xander into his slave, and during the night, he visits Buffy, bites and put her under his thrall. Buffy hides the bite with a scarf and becomes powerless and seduced by the dark prince, who promises to disclose to her the darkness of her powers and increase them.

After meeting Dracula, Buffy commits with Giles to be her watcher and help her to find the origin of her powers. Joyce Summers asks Buffy to look after her sister Dawn, who feels shadowed being the little sister of a famous slayer and an outsider of her group. They go with Giles, Tara and Willow to Mr. Bogarty's Magic Box store and Willow stumbles in his dead body. They realize that a gang of vampires have killed the man. Sooner they find that Harmony has formed a group of vampire minions with the intention of destroying Buffy. When Dawn invites Harmony to get in her house, the upset Buffy decides to chase Harmony and her vampires.

Xander is not satisfied living in the basement of his parent's house and is looking for an apartment of his own to live with Anya. Meanwhile, a demon called Toth chases Buffy at the Magic Box, and his smell guides Giles to the city dump. When Buffy and the Scoobies see Toth in the garbage landfill, he uses a rod device called Ferrula-Gemina blasting a ray against Buffy but hitting Xander instead. Toth vanishes and the group leaves the place with a confident Xander. Later the insecure Xander awakes in a pile of trash, follows his double and meets Willow to tell about his clone. Giles finds that the ray discloses one person into half, distilling personality traits into two separate bodies. Further, if one part is destroyed, the other will die. While trying to find a mean to unite Xander, they have to face the evil Toth.

Joyce faints while at home, and while the gang is at the hospital, Dawn discovers that Riley's heartbeat is off the charts, but Riley ignores it. When Buffy enlists the Initiative's help, Riley gets angry and runs, and he eventually admits to Buffy that he's afraid they'll make him a normal guy, causing her to lose interest in him. She manages to get him to the Initiative doctor, only to find that Spike has forced the doctor to remove his chip...

A mysterious woman arrives in town looking for "The Key", and Buffy investigates a possible supernatural cause behind her mother's illness. She performs a ritual to see spells, and discovers something is not quite right about Dawn. Meanwhile, the gang helps Giles see through the re-opening of the magic shop, and Giles hires Anya to work for him.

Buffy tells Giles the truth about Dawn and they decide to keep the secret about her origin. Buffy moves back home to stay together with her sick mother and Dawn. On the Eve of Tara's birthday, her big brother Donny, her father and her cousin visit her to bring her back home, since she would have a demon inside that would unleash in her twentieth anniversary. Meanwhile, The Beast forces Lei-Ach demons to chase and kill Buffy. Tara casts a spell to her friends to make demons invisible bringing trouble to her friends.

While patrolling the cemetery, the self-confident Buffy fights against a vampire, is stabbed with her stake and saved by Riley. She tells the incident to Giles and decides to research how the previous Slayers were killed. She does not find any information in the watcher's journals and decides to seek the information from Spike. He tells that in 1880, in London, he was a bad poet and a shy good man in a non-corresponded love with Cecily. When he meets Drusilla, she bites and converts him in the evil vampire. Later, in the same year in Yorkshire, he meets Angel and Darla and is challenged to defeat a slayer. In 1900, in China, he is well-succeeded and kills his first slayer. In 1977, in the subway of New York City, he kills his second slayer. He advises Buffy that she must never be reckless and keep her death wish to survive.

Joyce goes to the hospital for a CAT scan, the doctor finds a shadow and she is immediately submitted to a surgery. The doctor discloses that she has a low grade glioma (i.e. a brain tumor). Buffy wants to use magic to heal her mother, but Giles, Willow and Tara advise her that it would be of high risky for humans. Meanwhile, Glory goes to the Magic Box, and the naive Giles sells two powerful components for an ancient Sobekite spell. Anya discloses it later through the receipts, while Glory conjures a snake demon to track the Key. When the snake demon finds Dawn in the Magic Box, Buffy and Giles follow the demon trying to kill it before getting to Glory.

While Buffy and Dawn stay with Joyce in the hospital, Giles, Willow and Xander patrol the cemetery chasing vampires and Riley continues in his process of transformation in a vampire. Later, while watching the stars, Tara and Willow see a meteor crashing nearby Sunnydale. The Scooby group realizes that a demon called Queller has just landed on Earth. Joyce returns home since her surgery will occur only two days after, and the Queller arrives in the hospital attacking crazy people. Due to the compression of the tumor to her brain, Joyce acts strange having hallucinations, but she also sees the truth about Dawn and makes Buffy promise that she will take care of Dawn if she dies in the surgery. When the Queller arrives at Joyce's home chasing her, Buffy fights with it to save Joyce and Dawn.

After the surgery of Joyce, the surgeon tells Buffy, Dawn, Riley, Giles, Willow and Xander that the procedure was successful and the brain tumor had been totally removed, and Joyce would stay in the hospital for observation. While Anya and Xander lodge Dawn, Buffy and Riley spend the night together in her house. However, the snoopy Spike sees Riley leaving Buffy's place in the night while stalking her, and finds that Riley is addicted in vampire bite in a junkie nest where people pay to be bitten by vampires. He discloses the truth to Buffy, and she breaks with Riley after and argument. When the Initiative invites Riley to go to an operation in Belize, he gives an ultimatum to Buffy about the future of their relationship.

Giles travels to London for a watchers' meeting to discuss about Glory and leaves Anya in charge of the Magic Box. When Willow uses the supplies of the store to cast a spell to create a ball of sunshine to help Buffy to slay vampires, Anya argues with her and the distracted Willow releases a troll imprisoned in a parallel world. Olaf the troll destroys the store and goes to the Bronze to drink beer with the intention of eating babies. Meanwhile, the whimpering and needy Buffy is missing Riley and Willow and Anya chase Olaf. When they fight against the troll, Xander unsuccessfully tries to help his girlfriend and his best friend, and Olaf asks him to choose between Anya and Willow who might live.

Giles informs Buffy and the Scooby gang that the Council of Watchers leaded by Quentin Travers will come to Sunnydale with information about Glory. When the arrogant team arrives, they blackmail Buffy and her friends, threatening Giles to send him back to England. Buffy and her friends accept to be submitted to the review leaded by Quentin. However, when Glory comes to Buffy's home and threatens her family and friends and Buffy is attacked by the Knights of Byzanthium, she changes her attitude in a matter of power.

On the day of her twentieth birthday, Buffy tells the Scooby Gang that Dawn is The Key and they act in a suspicious way with Dawn. In the night, Dawn leaves the party and breaks into the Magic Box with Spike, seeking Giles's diary and finding the truth about her. Meanwhile, Glory destroys the Knights of Byzantium and somehow switches body's with Ben, fighting against Buffy and her friends. Willow uses a powerful teleportation spell to momentarily get rid of Glory.

While in the Bronze with Scooby Gang, Spike sees Buffy alone at a table and talks to her. However, Buffy sees Ben and stays with him. Meanwhile, a train arrives in Sunnydale with all the passengers dead. When Buffy sees that Dawn is hanging out with Spike, she asks if Dawn has a crush on Spike, and she tells that Spike is in love with her. When Spike confesses that he is in love with her, Buffy becomes nauseated, upset and rejects him. Meanwhile, Drusilla meets Spike, convinces him that the chip in his brain does not cause pain, and Spike is back.

Joyce is nervous and her anxiety is because she is going to date a man called Brian that she has recently met. Meanwhile, a strange and beautiful girl called April arrives in Sunnydale seeking her mysterious boyfriend Warren. Buffy and her friends are in a party, and when Spike flirts with April, she throws him through the window, and then hits Buffy. Buffy and the Scooby gang realize that April is a robot searching for her creator. Meanwhile Glory finds that Ben unsuccessfully tried to date Buffy.

Buffy sees Joyce lying on the couch, calls 911 and unsuccessfully follows emergency procedures. When the paramedics arrive, they realize that Joyce is dead. Buffy calls Giles and goes to the school to tell Dawn. Along the day, the Scooby gang grieves the death of Joyce and sympathizes with Buffy.

Buffy organizes the funeral services for Joyce without a wake, respecting her last will, and after the service she meets Angel, who has come to Sunnydale to support her. Meanwhile Dawn becomes obsessed in bringing her mother back. She steals infamous witchcraft books from Willow and from the Magic Box, and helped by Spike, they visit the demon Doc, an expert in black magic that teaches Dawn resurrection techniques. She steals a Ghora demon egg in the Hell Mouth with Spike for her black magic, and she interrupts her spell after a serious conversation with Buffy. Meanwhile Ben unintentionally discloses to Jinx that The Key is an innocent in a human form.

Buffy is feeling that she is turning into stone, losing her ability to love and her humanity, and wants to take a break in her vampire slayer's duties. Giles persuades her to travel for a vision quest in a sacred place in the desert to bring her concentration back. Meanwhile, Spike gets the Buffy-robot from Warren and both the Scooby gang and Glory's minions believe that the robot is the real Buffy, and the minions assume that Spike is The Key. Buffy has a vision of the First Slayer, who tells her that death is her gift. Spike resists Glory's tortures and receives his reward in the end.

While Buffy tries to control a motherless Dawn and bring their lives back to some sort of normalcy, Tara and Willow get into their first major argument. Meanwhile, Glory thinks she has figured out who the Key is.

Buffy realizes that she can never beat Glory, so she and the Scoobies leave Sunnydale in Spike's RV. Unfortuntely, the Knights of Byzantium are quickly on their tail, deciding that they must kill Dawn before Glory can get to her.

Despite the gang's best efforts, Glory has succeeded in taking Dawn. This failure after so much effort proves too much for Buffy and she slips into a catatonic state. Willow must use magic to enter her mind and help her work through the trauma before it is too late to save Dawn and the rest of the world from Glory.

In the 100th episode, another imminent apocalypse is at hand in Sunnydale. Buffy must square off against a God when Glory prepares to use Dawn to break down the walls between the dimensions and unleash all Hell on Earth. Buffy and the gang go to battle, knowing that not all of them will survive.