Buffy wakes up in Angel's bed at the mansion after the two fell asleep after patrol. Buffy talks a bit about bringing some of her stuff over so every once in a while she could spend the night and she mentions going to the prom. When she opens the curtains to check for the sun, she accidentally lets in a strong beam. It hits Angel, but he manages to move before it really burns him.
At school, Anya, still a mere human being, asks Xander to the Prom. Both are dateless and desperate, so he says yes. Later, Buffy, Willow, Oz, and Xander discuss prom plans, and dresses. Joyce visits Angel at the mansion and tells him she's worried about the effect he's having on her daughter. She tells him that he can't get in the way of her future and if Buffy can't see that, he'll have to do something about it.
Buffy and Willow are talking prom dresses while Giles is continuing research for the Ascension. Although the pages Willow tore out the Books of Ascension have helped, they don't know for sure what kind of demon the Mayor will transform into. Giles and Wesley will both be chaperoning the prom which Buffy protests is going to be the one and possibly last night of fun they'll have in Sunnydale. Meanwhile, in a small house, someone plays a video for a caged beast that makes it go wild.
Buffy and Angel are in a church getting married. As they exit the church, Angel is fearful of the sun, but it is Buffy that burns up and dies--not him--when they come in contact with the sun's light. Angel wakes up.
While patrolling, Buffy and Angel have an argument. Angel's still very disturbed by his dream, the visit from Joyce, and what the Mayor said to him and Buffy during their last encounter and tells Buffy they need to talk. He tells her that it's unfair for her to be in a relationship with him because of all the things he can't give her. In order to make her realize how serious he is, he breaks up with her and tells her he doesn't want be with her. He plans to leave Sunnydale after the ascension.
Buffy goes to Willow for comfort and she realizes that Angel was right; the two of them really can't be together. But, after such a long and dedicated relationship falls apart so quickly, Buffy just feels horrible and cries. At "April Fools," a store where Cordelia works, Xander spots her through the window and goes in to tease her for what he perceives as her spending a long time trying to pick a dress, however he is stopped short when an assistant chides Cordelia for speaking to a customer. She reveals that due to her parents committing tax fraud, she's now working there to save up for a prom dress because her family lost all of their money and now she has nowhere to live and she can't afford to go to any of the colleges that accepted her. Xander is clearly shocked by this and can't come up with any words in response while Cordelia says that he can go back to his friends and laugh at her getting her comeuppance. Their argument is cut short as one of the beasts that broke free breaks through the window and kills a boy dressed in a tux.
At the library, they study the video of the attack, and spot a guy outside who turns out to be Tucker Wells, who used to have a chemistry class with Oz. Tucker has plans to ruin prom night by sending a hell-hound trained to attack those in formal wear after the students. Buffy vows not to let the hellhounds ruin the big night and issues orders for everyone to split up and search for clues. She ends up finding an address at the butcher where Tucker ordered cow brains to feed the demons. She runs into Angel and the two have a post-break-up argument. As Cordelia leaves work, she finds that her prom dress has been paid for.
Buffy returns from her searching and orders everyone else to attend the prom while she takes care of the hounds. She tells Giles that Angel is leaving town. She finds Tucker in his basement and confronts him, asking him why he wants to ruin the happiest night of a senior's life to which Tucker simply tells her he has his reasons (although the audience see that it's just because the girl he asked to the prom said no). Buffy ties Tucker up before he can release the hound, but she also finds out that there are four hounds, and three are already on their way to the school. After killing all of them, Buffy changes into her prom dress and shows up for the dance. Everyone looks amazingly dressed up in their tuxes and formal dresses. Cordelia shows up and clings to Wesley (after thanking Xander for paying for her dress, restoring their friendship) while Anya tells Xander of her past cursing unfaithful men, and Willow and Oz just enjoy the evening.
When Class awards are given out, Xander is disappointed when he is snubbed for Class Clown. Then Jonathan approaches the microphone and asks if Buffy is in the room. Buffy, who was getting punch, slowly turns around. Jonathan says that the next award is a new category, and gives a speech saying that while they may not be friends, the student body has noticed whenever something creepy or horrible happened, Buffy was always there to stop it and that most of them have been saved by her at least once. As a result, the Graduating Class of 1999 has the lowest mortality rate in Sunnydale High history and she is to be given an ornately decorated parasol bearing a small engraving saying 'Buffy Summers - Class Protector'. Buffy can't believe what she's hearing, and approaches the stage to the cheering of her classmates and friends and she happily accepts the award from Jonathan. Afterwards, the students continue to dance when Angel surprises her by showing up in a tux. While Angel assures her that his presence doesn't change their status, Buffy is just glad to be held in his arms as they dance.