Buffy slays a vampire to save a teenage boy in an alley. The Scooby Gang, knowing that Glory has the advantage over them, make plans to try to avert the apocalypse that will result if her plans succeed. None of them have any ideas apart from killing Buffy's sister Dawn, which Buffy refuses to consider. Xander reluctantly suggests killing Ben as, although he is an innocent, his death will destroy Glory. Giles however points out that it's unlikely Ben will resurface this close to the ritual. In the end Anya suggests the use of two items that had been found earlier in the series: the Dagon Sphere, which is supposed to repel Glory, and Olaf's hammer. The Scoobies finally decide on a plan: as the Key can only be used at a specific place and time, all they have to do is keep Glory preoccupied and delay the ritual until her window of opportunity has passed, after which the Key will be useless to Glory and Dawn will be safe from her.
They then try to find out where Glory's plan might be occurring, and Tara, who was made insane by Glory in "Tough Love", tries to lead them there saying that she has a big day and needs to be somewhere. All of Glory's victims who had been turned insane instinctively knew where Glory was performing the ritual and were drawn there to assist. Thus, the Scoobies decide to simply follow Tara when the time comes.
Xander and Anya search for the Dagon Sphere in the Magic Box's basement; Xander proposes to Anya but she tells him to give the ring to her again, "after the world doesn't end." Buffy heads back to her house with Spike in tow. She invites him back into her house, and they both let one another know they respect each other. Buffy asks Spike to protect Dawn, and Spike tells Buffy he knows she'll never love him, but appreciates her for treating him like a man. Meanwhile, Dawn is tied up at the top of a tower that Glory's minions have constructed for the ritual to open the gates between dimensions, allowing Glory to return home but setting all hell loose on Earth in the process.
The group prepares to leave, waiting until sundown. As Tara arrives at the tower, she is accosted by Glory who demands to know what she is doing there. Glory is then ambushed by Willow, who performs a spell that restores Tara's sanity, confuses and dazes Glory having reduced her mental energies, and throws all three of them backwards violently. Glory, still dazed, decides that she needs a brain to psychically feed on and sees Buffy. She orders her minions to guard the path up the tower.
Buffy brandishes the Dagon Sphere and tosses it to Glory, who is obviously in pain holding it. Glory manages to crush it in her hands. In this time Buffy runs up and attacks her, initially gaining the upper hand. Giles, Spike, and Anya attack the minions at the base of the tower, with the hope of breaking through to rescue Dawn. Glory regains her senses during the fight with Buffy and throws a punch that is hard enough to knock Buffy's head off, revealing that she was actually fighting the Buffybot.
The real Buffy sneaks up behind Glory and strikes her with Olaf's hammer. Glory is thrown backwards into a wall by the force of the strike. Buffy then forces her way past Glory's minions and attempts to climb the tower. However, Glory again regains her senses and quickly catches up with Buffy. They fight on the path up the tower and Buffy desperately tries to climb higher, but is hindered by Glory, eventually resulting in both of them falling off the tower to the ground. The hammer lands several feet away from Buffy and Glory confronts her before she can retrieve it. Glory is knocked off her feet, however, by a wrecking ball operated by Xander. Buffy picks up the hammer and sets about beating a dazed Glory repeatedly with it.
Glory reverts back to the form of the mortal Ben, and Buffy demands he leave Sunnydale and never return. She then leaves to try and rescue Dawn. Giles approaches a panting Ben and suffocates him to prevent Glory ever returning, knowing that Buffy couldn't kill an innocent, even though Glory would ultimately possess Ben again and kill Buffy.
At the top of the tower, Dawn is nervously watching the battle raging below. She is surprised to see Doc at the top of the tower and, unaware that he is a worshiper of Glory, cries to him for help. He instead pulls out a knife and advances on her.
Giles, Anya, and Spike are not making any progress through the minions and fall back, until Spike notices that there is someone at the top of the tower along with Dawn. Willow communicates telepathically with him and tells him to run to the top of the tower and rescue Dawn, and then she and Tara use magic to knock all the minions down. Spike sprints to the top of the tower and when Dawn sees him she calls to him. Doc turns round and Spike advances on him. The fight is short-lived however, as Doc proves to be far stronger than suspected and throws Spike from the tower, giving himself time to initiate the ritual. He makes two shallow cuts on Dawn's stomach.
Buffy runs to the top of the tower and nonchalantly pushes Doc from the top of it. She unties her sister but some of Dawn's blood has already dripped down and activated the portal. It expands and all manner of demons emerge from it around Sunnydale. Dawn, knowing that the portal won't close until her blood stops flowing, plans to jump into the portal, believing that doing so would close it and save the world, even if she dies in the process. Buffy stops her; she then suddenly has a flashback of several points in the season--telling Dawn they both shared "Summers blood," her earlier proclamation that "the monks made [Dawn] out of me," and most significantly, when she met the spirit guide who told her that "Death was [her] gift." Buffy realizes that she and Dawn share similar blood, and that if she, Buffy, throws herself into the portal, it will close. After exchanging a few brief words with Dawn, Buffy throws herself into the portal, closing it. Her battered body is found at the bottom of the tower, as the sun is rising.
Buffy's speech to Dawn: "Dawn, listen to me. Listen. I love you. I will always love you. But this is the work that I have to do. Tell Giles... tell Giles I figured it out. And, and I'm okay. And give my love to my friends. You have to take care of them now. You have to take care of each other. You have to be strong. Dawn, the hardest thing in this world... is to live in it. Be brave. Live. For me."
Buffy's friends grieve — Spike weeps — and the last scene of the episode is Buffy's grave bearing the epitaph:
SHE SAVED THE WORLD