Helen's sister, Rita's, daughter, Erin, is getting married and the Morgendorffers are invited. Even better, Daria and Quinn get to be bridesmaids.
When Daria's getting fitted for her horrible dress, Brittany and Jodie walk into the shop, and we find out that Ms. Li is somehow getting money from putting on a bridal fashion show, and Brittany and Jodie are the organizers.
When the Morgendorffers arrive at the the Windsor Hills Resort (where the wedding is to be held) - which, as Daria pointed out, looked straight out of The Shining - its discovered that Helen and Rita had, and still have, a MAJOR sibling rivalry. So, those two spend the entire time insulting each other, as do Jake and Rita's most recent boyfriend (the last one was involved in a tragic bungee-jumping accident).
Meanwhile, Daria's met her Aunt Amy, who's pretty much exactly like her, so the two hit off quickly, and make great comments throughout the whole thing. An added plus was Daria's chaperone, Lerman, a depressed, cynical guy - well, Daria's male equivalent, I guess.
Quinn's busy talking to the other pretty bridesmaids and flirting with every guy in the entire reception except Lerman.
Back in Laendale, Mack and Kevin go to the bridal fashion show to surprise their girlfriends, and are mistaken for a gay couple. Later, Kevin nearly embarrasses Brittany to death, and she gets mad at him.
In the end, the groom gets weirded out and runs away, Helen and Rita get drunk and have an extremely emotional forgiveness-love scene, the minister and Quinn's chaperone, Garrett, get into a fight over her, Jake gets to play golf in the dark outside the resort, and Daria and Aunt Amy take off to get cheese fries.