As narrator Buddy Glass openly admits, "Seymour: an Introduction" is as far from a typical short story as you can get. It's more of a collection of anecdotes and thoughts than a structured narrative, and so we won't even try to break it down into structured plot stages. However, we can do these typical plot analyses for "Raise High the Roof Beam, Carpenters."
We start right when something is about to happen – in this case, Seymour's wedding. In this stage, we get a little bit of the background story of the various Glass family members.
That's a pretty major conflict, not just for the bride and groom, but for Buddy as well. As the sole representative of Seymour's entire family, he's left with the responsibility for explaining his brother's actions.
A series of events complicates the central conflict considerably. First of all, Buddy is now in a small, confined space with a group of people who strongly dislike Seymour. (The Matron of Honor seems like she's about ready to strangle Seymour bare-handed.) Then they find out that Buddy is Seymour's brother. Then a parade going by stalls them in incredibly hot, uncomfortable weather. Buddy's pleurisy (painful lung condition) is acting up. The story of Charlotte Mayhew comes out. Then they end up at his apartment, which opens a new can of worms (the photos on the wall, Seymour's diary, etc.).
After the Matron of Honor goes off – again – about Seymour being a schizoid personality or total nut job, Buddy finally cracks. He yells at her and defends his brother admirably. He's been trying to control himself throughout the story, so this is the blow-up we've been expecting.
This isn't so much a discrete stage as a series of general questions that have been building throughout the story. Why didn't Seymour show up at his wedding? How will it all turn out? What is the story behind Charlotte's stitches?
This is where a good deal of those questions we just mentioned are finally answered. The central conflict has been resolved in that Muriel and Seymour eloped happily ever after all. (Of course, we know that it's not going to last, since we've already been told that Seymour kills himself in 1948, just a few years later.) Seymour's diary reveals quite a bit about his character and his possible motivations. Finally, Buddy's re-telling of the story of Charlotte's stitches satisfies our curiosity (to some degree).
As Buddy stated earlier, this was a day for written discourse, and so it's fitting that the story ends by touching on the idea of communication, yet complicates itself with a sort of Zen Koan touch. We discuss this more in "What's Up With the Ending?"