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 and 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. 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 dog beast that makes the dog 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 awakes from the dream.
While patrolling, Buffy and Angel have an argument. Angel is still disturbed by his dream and the visit from Joyce, telling 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 is working, 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 because of her father 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 for teasing them all this time. However, 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 hellhound trained to attack those in formal wear after the students. Buffy vows not to let this dog 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 dogs. 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 ties him up before he can release the hound, but she also finds out that there are four dogs, and three are already on their way to the school. After killing all of the dogs, Buffy changes into her prom dress and shows up for the dance. Everyone looks amazing 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, and Willow and Oz just enjoy the evening.
When Class Awards are given out, Xander is snubbed for Class Clown. But Buffy is actually honored with the Class Protector Award. It turns out Sunnydale High students were actually aware of Buffy's actions when protecting them from the evils of the Hellmouth the whole time. She's given an ornately decorated parasol for the award. After she accepts the award, 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.
Info from Buffy Wikia