The narrator has issues with blind people. Not that he's ever met a blind person. But, he has seen them in the movies. In any case, he isn't looking forward to this blind man, Robert, his wife's "old friend," coming to visit (1.1). That's how "Cathedral" begins.
We learn that the blind man, Robert, and the narrator's wife have been corresponding for the past ten years, revealing to each other the most intimate details of their respective lives. It's obvious that the narrator is jealous of Robert. That's the conflict, and that's maybe also part of why the narrator has issues with blind people.
When Robert arrives, things are a little awkward. The narrator isn't sure what to say to Robert. The woman is sure that every word the narrator says is a veiled blind joke, even though he's actually being nice to Robert. The complication is the simple fact that three people are hoping to get along and have a good evening. Because they all have baggage, and linked histories, the situation feels complicated.
Actually, we could say that all three make up the climax of "Cathedral." The climax is the stage of the story where the characters emotions are at a high level. In "Cathedral," as the characters connect through what some might call hedonistic, or indulgent activities. The connection is rather fraught because of the loneliness and dissatisfaction and personal issues each of the characters is experiencing. The combination of warmth and discomfort contributes to the sense that something is going on which is bigger than it appears.
The narrator's wife falls asleep, leaving the narrator and Robert alone. Will Robert and the narrator find a way to connect? And most importantly, will the narrator find a way to describe a cathedral to Robert?
The dénouement stresses the deficiencies of eyesight. When the narrator's wife wakes up and sees Robert and the narrator drawing the cathedral, she can't process what she sees. Nothing mysterious is going on. It's obvious what they are doing. But, it's unusual – it deeply challenges her ideas about what Robert and her husband would do if they were left to their own devices. By closing his eyes, at Robert's insistence, the narrator admits that being able to see might actually be limiting his experience.
Simple, but intense. Now that narrator has experienced seeing without his eyes, he feels free. We don't know if the feeling will last, if he'll continue his relationship with Robert, if his relationship with his wife will improve, etc. For now, the important thing is the moment, and both the complexity and the simplicity of his experience.