Buffy and Faith meet a demon (named Skyler in the script, though his name is not spoken) who offers to sell "the Books of Ascension" for $5000. When Faith reports this the Mayor, he asks her to kill the demon and take the books.
At the library, the Scooby Gang discusses "ascension" but they know very little about it until Willow suggests checking in a book which Giles was trying to hide from her due to its magical content.
Faith finds the demon, who is desperate to get out of Sunnydale before the Mayor's ascension, and kills him. She goes to Angel, saying she is upset over what she has become, and tries to seduce him so that he will lose his soul through happiness, but she cannot distract him from his devotion to Buffy. The Mayor therefore hires a mysterious cloaked sorceror to take Angel's soul in the "most painful way imaginable."
According to Whedon, fans had been eager to see Angelus again.
Xander gets the demon's address from Willy; Buffy goes to find him, and Faith tags along. They find the demon dead and Buffy is worried by Faith's strange behavior about it. Buffy tells Willow that she saw Faith kiss Angel; Willow urges Buffy to talk to Angel about it.
Faith approaches Angel at his mansion, on the pretext of apologizing for her behavior the night before. She pours blood onto his shirt; the cloaked man then steps out of the shadows and chants a spell; lights flash, Angel yells, and then Angelus is back. He pulls Faith to him and they kiss before Angelus starts beating her up. The two fight and Faith ends up on top, holding a stake to his heart. A truce is made as Angelus agrees to meet the Mayor and the two kiss.
Willow reports to the Scooby Gang that the Mayor's computer files were emptied before she was able to access them. With both Faith and Angel missing, Buffy leaves to find out what's going on with the Mayor while Wesley takes a very eager Cordelia and the rest of the group, minus Xander, to the Hall of Records to find information on the Mayor.
Faith takes Angelus to meet with the Mayor, and Angelus learns that the Mayor can't be killed. He tells the Mayor of his plans to torture and kill Buffy. Angelus and Faith walk to Buffy's house; on the way they meet Xander, whom Angelus knocks to the ground with a punch. Angelus and Faith lure Buffy to the mansion, where they reveal that they're fighting for the other side now. Angelus knocks Buffy out cold.
Buffy wakes up chained to a wall; Angelus and Faith plan to torture her. Faith talks about her painful childhood with a mother who drank too much, and her life as a Slayer living in Buffy's shadow. She reveals what she knows about the Mayor's master plan, which will culminate in his Ascension on graduation day.
Having learned all that Faith knows, Angel then reveals that his disensoulment was a sham, and Buffy shows that her hands were not really chained. The rest of the Scooby Gang arrive as Buffy and Faith fight and end up with knives at each other's throats. Telling Buffy that she can't kill her without becoming her, Faith breaks away and runs off.
At the library, Giles thanks and bids good-bye to the cloaked man, who faked the spell to repay an old favor: Giles had introduced him to his wife. Wesley is angry that Giles arranged this behind his back, and plans to tell the Watchers' Council about it. Now that everyone is fully aware of Faith's switch to Team Evil, they have to prepare to fight her as well.
Buffy and Angel are both shaken by what they had to do; but as Buffy leaves Angel's mansion, he asks, "You still my girl?" and she responds, "Always."