Buffy is training Dawn. Sunnydale High has been rebuilt, over the Hellmouth. And Dawn has to go there, since Buffy can't afford to move to another catchment area, or send Dawn to a private school. Dawn soon discovers there are evil spirits in the basement -- but who summoned them? She uses her new secret weapon -- a mobile phone to call Buffy for help. During the fighting Buffy discovers another occupant of the school basement -- a maddened Spike, hearing voices about how things are going to get worse. Anya is taunted for being too soft as a vengeance demon. Buffy accepts Principal Wood's offer to be an (unpaid) student counsellor. Meanwhile, in England, Giles and the Coven help to train Willow in her magic. She feels the active Hellmouth through her connection to the ground.
A woman walking her dog is attacked by a giant worm. This turns out to be the result of a wish granted by Anya. Spike turns up and offers to help. Anya realises Spike has his soul, and he fights her to stop her telling the others. Xander persuades Anya to reverse the spell, at just the moment Spike is stabbing the demon. He stabs the man, and his chip kicks in. Buffy discovers Spike has his soul back. There are lots of doom-laden prophetic words and dreams: Beneath you, it devours. Meanwhile, in England, Giles persuades Willow to return home, where she will be needed.
Willow returns, but she can't see her Buffy, Dawn, or Xander, and they can't see her. Mad Spike has a weird conversation with people who can't see each other. Anya tells Willow that being a vengeance demon is no fun any more. A flayed body is discovered, and the gang wonder whether it might be Evil Willow back. But it is a flaying demon, and the gang accidentally trap Willow in the cave with it. It paralyses her, and starts slowly flaying her. Anya tells the gang about Willow, and they realise they've trapped her in the cave. They race back in time to save her. The spell stopping them seeing each other dissolves, and Willow realises she had accidentally cast it, because she didn't know if she was ready to seem them again. Willow draws power from the earth to heal herself, but it's not enough. Buffy offers some of her own power to help.
Buffy starts her job as student counsellor. One student, Cassie, has given up, as she knows she will die next Friday. There is evidence this student can see the future, and Buffy is determined to protect her, with help from Dawn and the rest of the gang. Buffy saves her from a ritual demon slaying, and then from a crossbow bolt, but Cassie then dies of a heart condition. On the way, she has sprinkled foreshadowing comments.
Anya summons a spider demon who rips the hearts out of ten students tormenting a woman. Willow discovers the bodies, and protects herself from the demon with a hint of her dark power. Buffy decides she must kill Anya. We get flashbacks to when Anya was made into a demon in the first place [complete with bad translations from the Swedish]. Willow contacts Anya's demon master, who asks what Anya wants. She asks for the deaths to be reversed. He says the price is the life and soul of a vengeance demon, but Anya says she is willing to pay that price. So to her horror he kills her demon best friend.
Spike moves in with Xander. Dawn falls hopelessly in love with the new quarterback, LJ. When he is seen by counsellor Buffy, she falls, too. The rest of the gang realise it's a love spell, and try to figure it out. Xander and Spike discover it is the football jacket, but not before Anya and Willow fall under the spell. Willow decides to use her magic to turn LJ into a girl; Buffy goes off to kill Principal Wood for him, whilst Dawn decides to commit suicide on the railway lines. Xander and Spike just manage to stop all this, then steal the jacket and burn it. The recovered love-smitten are all very embarrassed.
Andrew and Jonathan return to Sunnydale to search for a dangerous artefact under the school. But Warren is telling Andrew to sacrifice Jonathan, so that the two of them can get god-like powers. Tara, through Cassie, tells Willow not to use any magic again or she will kill all her friends. When Willow says she doesn't know if she can do that, Tara suggests suicide as the only option. Dawn has a horrifying encounter in her own house, then her mother appears, and says Buffy will desert her. Buffy has a long conversation with a psychology-trained vampire, before he says Spike sired him. And we see Spike biting a woman. Willow discovers that she isn't talking to Tara, but to the creature from below -- are all the other encounters disinformation too?
Everyone slowly recovers from their eventful encounters. They all realise they may not have spoken to who they thought, but no-one is sure if they were actually lied to. Dawn is particularly quite. Buffy follows Spike to see if he really has reverted to his former behaviour. He has, but only under the manipulation of the evil below. Giles finds friends dead in London, and is in danger himself.
The First, as Warren, sends Andrew to buy pig's blood to complete the sacrifice. Willow catches Andrew buying blood, and takes him back to the gang. Principal Wood discovers Jonathan's body in the school basement, and secretly buries him. Spike is taken prisoner by The First, and used to complete the sacrifice to raise the Uber-vampire.
Spike is still a captive of The First, who is torturing him -- it is only Buffy's belief in Spike that keeps him defiant. Giles returns, with three potential Slayers -- most of the rest of the Slayers-in-training have been killed, and the line is in danger of dying out. Willow tries to find The First with magic, and is almost taken over. Giles and Buffy go hunting for The First, and Buffy fights the Uber-vampire, discovering it is stronger. The potentials are scared, and one runs off, but is killed by the Uber-vamp. Buffy follows, and is very nearly killed herself. Everyone wonders what to do next, then Buffy rallies them with a call to take the fight to the enemy.
The First tries to convince Spike that Buffy will never come for him. More potential slayers arrive in Sunnydale. They are worried they will all be killed -- especially after The First infiltrates them disguised as an already dead Potential, and spreads doubt. Kennedy is the only one of them who seems to want to try. Giles and Anya go to a demon dimension where they are told that The First has exploited an imbalance in the slayer power caused by Buffy being brought back from the dead. Buffy decides the new slayers need to learn a lesson, so arranges a big showdown where she kills the Uber-vampire in front of them all. Buffy rescues Spike.
Spike helps Buffy train the potential slayers to fight the First. Their "graduation" finds them locked in a crypt with a vampire. The gang learns there's another Potential in Sunnydale, so Willow casts a locator spell. It appears to show Dawn, but actually it was just passing through her on the way to the actual Potential. Dawn goes to fight a vampire because of it, and the real Potential gets to slay it.
Giles takes the Potentials to the desert to commune with the first Slayer. Willow kisses Kennedy, and transforms into Warren. She and Kennedy go to the local Wiccans for help, and find Amy. Amy pretends to help, but Willow takes on Warren's manner as well as appearance, and rushes off. Kennedy discovers Amy put a hex on Willow. Spike's chip starts to malfunction, so Buffy contacts Riley, who sends the Initiative to help -- by fixing or removing the chip; it's Buffy's decision which. The gang hear from Robson in London, and decide that Giles is The First, then discover he isn't. Willow starts to play out Tara's death scene as Warren, threatening Kennedy, but Kennedy transforms her back by kissing her again.
Spike is now chipless. Giles explains how he narrowly escaped death in London. Xander goes on a date, but naturally it's with a demon, who is trying to raise another User-Vamp. The latest Potential is Chinese, and speaks no English. Buffy goes to dinner with Principal Wood, and discovers he is the son of a Slayer, also engaged in the fight against Evil. The First tries to get Andrew to shoot the Potentials, but he instead tries to trap The First into revealing a weakness. Buffy, Wood and Spike rush to help Xander, and fight the demon together. Giles lectures the gang, saying they must not go on social dates, but focus on the coming fight. The First tells Wood his mother was killed by Spike.
The First persuades one of the Potentials to commit suicide. Buffy lays into the rest. Principal Wood gives Buffy his mother's Slayer Emergency kit, which holds a portal to the time of the First Slayer. Spike reverts to his long coat to slay a demon. Buffy meets the men who created her, and they offer her a demon's heart for more power. She refuses, and they show her the Hellmouth to come. Willow reopens to portal to get Buffy back, but drains Kennedy's lifeforce for a while, which freaks her out.
Andrew starts to film Buffy and her exploits for posterity. Sunnydale High experiences an outbreak of violence and hysteria: the Hellmouth is active early this year. Xander and Anya have one last fling. Kennedy is back with Willow. The gang realise Andrew can break the Seal of Danthalzar spell, and close the pentagram for a while. Buffy convinces him only his blood will do, and he breaks down, crying that he killed his best friend. But it's his tears that are needed, and the opening is resealed.
Willow gets a call for help from Fred. Giles tries a device to break Spike's trigger, flashing back to when he was sired, and then sired his own mother because he still loved her. Giles and Wood decide Spike must die, so Giles distracts Buffy, with talk of her being a General and having to make difficult decisions. Wood confronts Spike. Spike wins the fight, and threatens to kill Wood next time he comes for him. Buffy tells Wood that Spike is their best fighter, and she won't stop him from killing Wood if necessary.
Willow returns, bringing Faith with her. A raving priest, Caleb, stabs a girl, and uses this to send a message to Buffy. Faith and Spike each discover the other is now one of good guys. Buffy takes the Potentials to the Vinyard to fight Caleb, but he exhibits super-strength. He knocks out Buffy, kills some Potentials, badly injures some others, and puts out one of Xander's eyes.
Caleb appears to Buffy in the school, and knocks her out again -- goading her as part of some deeper plot. Giles discovers something about Caleb's past, and sends Spike and Andrew to investigate. They discover Caleb found an inscription that displeased him. Faith takes the Potentials for a night out at the Bronze. A group of police arrest her, and start beating her up. She fights back, and the Potentials come to her rescue. Buffy tries to rally the Potentials for another attack on Caleb, saying the Vinyard must be the source of his power. They revolt, as do her friends, and throw her out in favour of Faith.
Spike and Andrew return, and Spike goes to look for Buffy. The First appears to Faith as the Mayor. She and Robin comfort each other about their First experiences. Buffy is comforted by Spike. Kennedy and Willow comfort each other. Anya and Xander have a final final fling. In the morning Faith leads the Potentials into the sewers, looking for the First's arsenal. A revived Buffy confronts Caleb, and taunts him, rather than fighting. She discovers a secret cave with a special weapon. Faith discovers a load of Bringers, and then a bomb about to explode...
Buffy "King Arthurs" the weapon out of the stone, to Caleb's surprise. The First tells Caleb to let Buffy go for now. Faith and the Potentials are blown up, and the survivors struggle free of the rubble, only to be faced with several UberVamps. Buffy arrives, and finds her new scythe kills them easily. Anya and Andrew raid the hospital for medical supplies to treat the injured Potentials. Buffy gets Xander to take Dawn to safety, but Dawn revolts, and turns back. Buffy and Spike have a tender scene about the previous night. Caleb merges with The First to get more power. Buffy finds an old tomb, with the last Guardian, ancient makers of the scythe, and watchers of the Watchers. Caleb kills the Guardian, and he and Buffy fight. Angel comes to help, and watches Buffy strike down Caleb. They kiss, watched by Spike and The First.
Caleb rises again, so Buffy cuts him in two. Angel gives her a medallion of power for an ensouled more than human being -- Spike -- then goes back to LA to start a second front. The First appears to Buffy, and mocks her for being the one, always alone. Buffy realises this is just a rule made up by a load of dead men, and decides to change it. She gets Willow to cast a spell using the scythe, to change all the Potentials into Slayers. Willow is scared she will go dark, but she turns white. Down into the Hellmouth, the Potentials fight back the UberVamp army. Spike gets caught in sunlight, and the medallion comes to life, destroying the vampires, and imploding the Hellmouth. Anya is killed saving Andrew's life. Everyone else survives. Now there is a world full of Slayers, and Buffy is alone no longer.