Buffy and Willow are horrified at what they see: themselves in the mirror, wearing snot-green bridesmaids' dresses. And last night's rehearsal dinner was a nightmare! Bride-to-be Anya tried to pass her demon family off as "circus people." Groom-to-be Xander's family got totally toasted. But undaunted, Anya and Xander get ready for the wedding he's convinced nothing on earth can stop - except, maybe the Creepy Old Man who emerges from a portal. But, of course, he's not exactly of this earth.
Sunnydale's Bison Lodge is all spiffed up for the ceremony. Buffy helps a nervous Xander put on his cummerbund. Meanwhile, Tara and Willow flirt a little as they help the once-demon bride hook up her wedding dress. Anya is excited about being with her best friend Xander forever.
Guests gather. Xander's drunk and lecherous Uncle Rory is certainly filled with spirit - and spirits. Vengeance demon D'Hoffryn offers a "Hymen's greetings" to all assembled and presents Dawn a wrapped gift - with squirmy tentacles sticking out! Even fellow vengeancer Halfrek asks D'Hoffryn to chill. Spike shows up with a skanky Goth Girl as his date. They mix with Anya's family - like Loose-Skinned Clem, Tentacle Demon and Warty Demon. Of course, tension is building. It's not easy mixing freaky demons with alcoholic humans. And when Dawn tells Buffy that Spike has brought a "ho-biscuit" skank to the wedding, a jealous Buffy looks like she could use a cocktail herself. And just why is that Creepy Old Man here? To urge Xander not to marry Anya, that's why. Then he adds that he, the Creepy Old Man, is Xander - from the future!
As demon-human tensions rise, young Xander follows "old Xander." He gets sucked into a mystical orb the Old Man produces- and into his future. Xander sees himself as a jobless couch potato with two kids and a Mary Kay cosmetics-selling wife. After a flash-forward peek at thirty miserable years of marriage, Xander is thrust back into the present. The future-Xander begs the now-Xander to call off the wedding. At the same time, Spike and Buffy exchange looks. Spike clearly wants to be with the Slayer, not the skank. But later for that. As the band starts to play "Here Comes The Bride," there goes the groom. Xander bails.
Buffy stalls for time. The Warty Demon even tap-dances. But the natives are restless. Demons blame humans for the delay, humans blame demons, so when Dawn finally tells Anya that her groom is gone, push comes to shove. It's one big can of whoop-ass as the place erupts into a Human vs. Demon Smackdown!
Anya finds out Xander had been talking to the Creepy Old Man. When she confronts him, the creepy geezer peels open his true self: an enormous demonic Creature. He says Anya did this to him, and swipes his claws at her. The Creature confesses he was human once. Years ago a woman he was dating - and cheating on - angrily summoned a vengeance demon. Anya appeared, and turned him into this grotesque creature, forever tortured in another dimension. Ruining Anya's wedding is his revenge. Adding injury to insult, he claws the abandoned bride.
Suddenly, Xander appears heroically to the rescue. But it's the Slayer who actually kills the Creature, of course. Anya tells Xander that the Geezer-Creature lied to him about having a miserable married future, but he's not so sure. Xander is afraid he and Anya really could become stale mates. They leave Bison Lodge - but not together. Buffy's bummed. Thinking of Spike, she had high hopes for a loving Human-Demon marriage. Xander checks into a crappy motel. Alone. In her apartment Anya is comforted by D'Hoffryn. The fellow Vengeance Demon tells Anya that becoming domesticated has robbed her of her true powers. Temptingly, she adds that maybe it's time for Anya to go back -- from domestic to demonic. And Anya thinks about it.
Summary from Buffy.com