Dudley Dursley reluctantly and somewhat awkwardly accepts an invitation to his cousin's wedding. While there, he meets Harry's friends, discovers some fascinating wizarding adult beverages - and meets Cho Chang, who has her own reasons to find herself out of place and drinking heavily at the wedding of Harry Potter and Ginny Weasley.
In the months to come, this unlikely encounter will make him rethink his attitudes towards the magical world, drastically change the course of both of their lives, and change Dudley's relationship with his family forever.
JK Rowling once said in an interview that she thought about having Dudley have a magical child on platform 9 3/4s at the end of book seven, but decided that no magical blood could survive contact with Vernon Dursley's DNA. In a long list of ideas she got wrong about what should happen to her characters post-canon, I've always thought that this was pretty near the top.
Despite my reservations, this mostly takes the Harry Potter epilogue as written, but makes a few significant changes that the plot revolves around.
CW: vomit
Apologies, I've been lax about moving the rest of this story here. Working on getting the rest up!