In the season premiere, the famous vampire Count Dracula makes an appearance in Sunnydale, in search for Buffy. He then bites Buffy in the same place where she was bitten by The Master and Angel. Buffy breaks free from his will and Dracula is defeated and staked
Season five also introduces a younger "sister" to Buffy, Dawn, who suddenly appears in Buffy's life, and although she is new to the series, to the characters it is as if she has always been there. Due to problems with her mother, Buffy suspects Dawn maybe harming Joyce. However they discover that Joyce has a brain tumor. As a result Joyce sees Dawn for who she is, a beautiful supernatural presence. Upon learning of Joyce's tumor, Buffy leaves her dorm to take care of her mother
Meanwhile, Glorificus, a hell-god that was exiled from her dimension after a war, has been freed from her human form, known as Ben. Glory, taking the form of a beautiful but vain young woman, with her powers still intact but she must drain people of their sanity. She begins searching for a "Key" that will allow her to return to her Hell dimension and in the process would blur the lines between dimensions and unleash Hell on Earth. It should be noted that at the beginning of the season Glory and Ben are vying for control, alternating who controls the body they share; later the line between them begins to blur and they experience each other's emotions and memories.
It is later discovered that the Key's protectors had turned the Key into human form as Buffy's sister Dawn, concurrently implanting everybody with lifelong memories of her and creating a body biologically related to the Summers family. The Watchers' Council, who Buffy previously cut ties with, aids in Buffy's research of Glory, and she and Giles are both reinstated by the Council.
Riley leaves early in the season after deducing that Buffy does not love him and joins a military demon-hunting operation, while Spike, still implanted with the Initiative chip, realizes he is in love with Buffy and continually helps the Scoobies in their fight. Buffy continually refuses his advances and alienates him.
Xander's girlfriend Anya Jenkins gains more human feelings, both negative and positive, such as a love for money. Anya is hired by Giles and works alongside him at The Magic Box.
Glory attacks Willow's girlfriend Tara Maclay, draining her of her sanity. In a rage, Willow attacks Glory and in the resulting fight, Willow is nearly killed if it were not for Buffy's intervention.
Spike meets Warren Mears, to build a robot to Buffy's likeness. This greatly angers Buffy, but she soon softens after Spike is tortured by Glory where he does not reveal who the Key is. After this ordeal, Buffy warms up to Spike, promising she won't forget what he has done for her and invites him back into her life.
Near the end of the season, Joyce dies of an aneurysm, devastating Dawn and Buffy. Glory later discovers that Dawn is the Key and kidnaps her. After Xander proposes to Anya, Buffy and her friends track Glory and Dawn down to a tower built by Glory's minions. At the time of the ritual, Glory uses Dawn's blood for a ritual but is distracted by the Scoobies intervention. Willow regains Tara's sanity, severely weakening Glory. After overpowering Glory, Buffy tells her to leave Sunnydale or die. When Glory reverts back to Ben, Giles kills Ben to prevent her return. Dawn becomes willing to sacrifice herself to save the world, but Buffy realizes that because she is related to Dawn, she can also stop the apocalypse and realizes what the First Slayer's message of "Death is your gift" and sacrifices her own life to save Dawn's and prevent the portal to the Hell dimensions from opening. Buffy's friends mourn her death and praise her for saving the world.