Giles confides in Willow -- Buffy doesn't need him any more, so he's going back to England. Dracula comes to town to seduce the famous Slayer, who has been exhibiting a darker side since the evocation of the First Slayer in " Primeval ". He nearly succeeds, biting her, and getting her to taste his blood. But Buffy breaks free in the end and stakes him -- twice. She confesses to Giles that she needs his help to explore this darker side. Then she goes home and finds her sister...
Buffy is irritated by constantly having to look after her younger sister, Dawn, who everybody seems to know. The gang discover the magic shop proprietor has been murdered, and Giles decides to take over the shop. He explains it's just like a library, except that people pay for the things they don't bring back. Tara seems uncomfortable when Willow calls her "one of the good guys". Harmony and her new gang of vampires are out to kill Buffy, and capture Dawn to lure Buffy into a trap. Spike told them it wouldn't work...
A demon, while trying to destroy Buffy, accidentally creates a double of Xander. The double is cool and confident, and convinces the gang the real Xander is a demon. The real Xander, even more bumbling than usual, convinces Willow that the double is the demon. But then Giles discovers neither is a demon, that Xander has been split into two components of his personality, and if one is killed, both will die.
Riley seems super-strong, helping Buffy kill vampires. Buffy gets a new training gym behind the magic shop. Joyce looks at Dawn, wonders who she is, and passes out. When she recovers she has no memory of her question. But visiting her in the hospital, they discover Riley is sick -- his heart is racing, making him feel powerful, but he won't be able to keep up the pace much longer. Buffy eventually persuades him to go to a doctor -- just as the doctor has been kidnapped by Spike and Harmony, to remove the chip in his brain. The doctor pretends to, but Spike finds he is still restrained when he tries to bite Buffy. The doctor cures Riley, who is now worried Buffy won't love his "normal" self. Spike wakes up from a horrifying dream about Buffy...
Buffy discovers a strange orb linked to some monks. The magic shop is a great success, and Giles offers Anya a job in it. Trying to find out more about her mother's continuing illness, Buffy goes into a trance of clear vision. She finds out nothing about Joyce, but realises Dawn is not her sister. Spike is languishing outside Buffy's house. Buffy confronts the last monk, who has been tortured by an ancient evil creature -- who looks like a beautiful young woman -- looking for "the key". With his last breath, he tells Buffy that Dawn is that key made human, and sent to the Slayer for safety -- and that Dawn does not know this.
Buffy moves back home to protect Dawn -- she tells Giles, but they decide not to tell the others. Tara's family come to town to take her home before her fateful 20th birthday, which will reveal her secret -- that she is part demon. In desperation, Tara spells her friends to stop them seeing her soon to be exposed demon part -- but they become blind to all demons. Including the ones trying to kill Buffy to clear the way to Dawn. Horrified, Tara realises what she has done, and removes the spell just in time. Spike proves that Tara is human -- the story is a family legend used to control "the womenfolk". The gang refuse to give Tara up to her kinfolk.
Buffy is badly hurt trying to stake a vampire, and survives only because of timely intervention by Riley. She decides she needs to know more about how Slayers die. There is nothing documented, so she pays Spike to tell her about the two Slayers he killed -- and we learn the surprising reason for his name "William the Bloody". Meanwhile, Joyce goes to hospital overnight for tests.
The ancient evil being, Glory, buys some amulets at the Magic Box that enable her to transform a cobra into a monster snake to seek out the key. Buffy, devastated to learn how ill her mother really is, and looking for a fight she can actually win, takes on Glory, but gets pounded. At least she can take out the snake before it reports Dawn's whereabouts. Meanwhile a depressed Riley, confused by some taunts from Spike, is haunting vampire bars.
While Buffy is distracted waiting for her mother to have brain surgery, a deadly Queller demon comes down to Earth. Riley calls in his military friends, and the Scooby team discover the Queller demon has come to clear up bouts of madness throughout history. Five patients die in the mental ward at the hospital, then the demon vanishes. But it has followed Joyce home, and is about to kill her, too. Buffy only just manages to kill it first. Dawn is distraught at the number of mad people who tell her she doesn't exist.
Joyce's surgery goes well, the doctor says. Spike takes Buffy to a vampire nest, to show her of Riley's visits to the vampires. Riley explains it is because she doesn't need him, and issues an ultimatum -- either she gives him a reason to stay, or he will go with an Army team deep undercover to destry demon nests. She reacts angrily -- but Xander gives her a stern talking to, making her realise what she is letting go. She dashes off to Riley -- but arrives just too late.
Giles goes to the Watchers' Council to learn more about Glory. Willow and Anya run the magic shop, but while squabbling, accidentally materialise a Troll who used to be Anya's boyfriend before she became a vengeance demon. Spike is still pining over Buffy, and tries to gain her approval. Willow and Anya's problems over Xander are resolved in the fight against the Troll. Dawn overhears some of Giles and Buffy discussing her.
The Watchers' Council comes to Sunnydale to check up on Buffy. They have information on Glory, but will give it to Buffy only if she can pass their "tests". Glory comes to Buffy's house and threatens her and her family, demanding the "key". Buffy takes her mother and Dawn to Spike for safe-keeping. She realises she has power over the Committee, and demands the information from them. They tell her that Glory is more than just a demon...
On Buffy's 20th birthday, Dawn learns her true identity, and runs away. She meets up with Ben in the hospital, and he realises she is The Key. He is desperate for her to leave before Glory does, and then he transforms, into Glory! Glory, who does not remember the conversation with Ben, tries to get Dawn to tell her where Buffy has hidden the Key, and Dawn manages to fool her long enough, until the gang arrive. Again, she is too strong to be beaten, but Willow manages a dangerous teleportation spell. Dawn doesn't remember that Ben became Glory.
Dawn hangs out with Spike because he is cool. Buffy learns that Spike has a crush on her, and is revolted. (At least she didn't discover he was playing Slayer games Harmony!) Drusilla arrives in town, to turn Spike back into a bad boy. But he still loves Buffy, and Dru goes away, disgusted. But Buffy is still disgusted, too.
A female robot built to be the perfect girlfriend comes to Sunnydale, looking for her creator Warren. But Warren has dumped her because she is too perfect, too predictable. She causes mayhem looking for Warren, and Buffy berates Warren for being so cruel to her, not at all aware of the analogy with her behaviour to Spike. Glory discovers a message from Buffy on Ben's answerphone. Spike asks Warren to build him a Buffy-robot. Buffy discovers Joyce collapsed at home.
Joyce unexpectedly collapses and dies of a brain haemorrhage. Everyone is devastated. [A brilliant, if harrowing, episode]
While Buffy tries to cope with the funeral, comforted by Angel, Spike discovers Dawn attempting a spell to bring back Joyce. He helps her, and the spell seems to work. Glory discovers the key is a person. At the last moment Dawn realises how much it would devastate Buffy, and stops the spell.
Glory's hench-demons see Spike with his robot-Buffy, and believe he is the Key. They take him to Glory, but she knows the Key has to be pure, so cannot be a vampire. She tortures him to find the real Key, but he refuses to tell her. The gang think robot-Buffy is the real Buffy. Meanwhile, Buffy is on a spirit quest in the desert, to find some answers about love. She gets back just as the gang are off to rescue Spike. Buffy is appalled by the robot, but touched by Spike's refusal to tell Glory about Dawn.
Dawn has been skipping school, and Buffy is warned she won't be allowed to be Dawn's guardian unless she can control her. Willow and Tara quarrel. Glory decides Tara is the Key, and is bitterly disappointed to learn she isn't, and demands Tara tell her who is, When she refuses, Glory puts her glowing fingers in Tara's brain, leaving her deranged. A distraught Willow attacks Glory, and has to be saved by Buffy. Then something the babbling Tara says makes Glory realise that Dawn is the Key.
Buffy realises they can't fight Glory, so the gang leave town. But the Knights of Byzantium are on their trail, and attack their Winnebago. Giles is badly hurt, and they hole up at an abandoned gas station, besieged by the Knights, but protected by Willow's barrier spell. Tara seems somehow connected to the other victims of Glory, who all break out of the psychiatric ward. The Knight's leader tells Buffy that Glory wants the Key to get back to her own dimension, but that the Key will open all portals, mixing all the dimensions, causing chaos and the end of the universe. The Knights agree to medical help for Giles, and Buffy calls Ben. But after he has helped Giles, he transforms into Glory, who grabs Dawn, kills all the Knights, and runs. Buffy collapses.
Willow enters a catatonic Buffy's mind, to try to bring her out of herself. Glory finds the barriers between her and Ben slowly crumbling, so now she is experiencing his feelings. Only Spike can remember that Glory and Ben share a body. Willow finds Buffy caught in a loop where she keeps killing Dawn. Willow persuades her to break the loop, and Buffy emerges from her funk, only to be told by Giles that the only way to stop Glory is to kill Dawn...