Most good stories start with a fundamental list of ingredients: the initial situation, conflict, complication, climax, suspense, denouement, and conclusion. Great writers sometimes shake up the recipe and add some spice.
Exposition (Initial Situation)
In the Garden
We meet our six narrators when they are children in a nursery school together, spending their days in lessons and frolicking outdoors in the garden like (excessively morbid) little lambs. In this section, we begin to learn about the protagonists and their dynamics with one another. This phase of the book may represent a hat tip to the Biblical tale of the Garden of Eden, which also described a universe in infancy. Basically everything is sunshine and rainbows, with a little side of death-obsession.
Rising Action (Conflict, Complication)
School Daze
During this phase of the narrative, the six narrators separate down gender lines to go to their respective boarding schools. It is at this point that the boys meet Percival and Neville falls in love with him (we also meet Dr. Crane, but only Louis likes that dude). The protagonists then graduate and get into their adult careers (with a stop at university along the way, for some). Percival (who was at some point introduced to the girls as well) goes off to India to work for the imperial government there.
Climax (Crisis, Turning Point)
Just When We Were Starting to Have Fun, Here's Death
The six protagonists are heartbroken when they learn Percival has fallen from his horse in India and died. Percival's death throws the characters into a tailspin as they try to make sense of this violent interruption in the narrative of their lives. Everybody loved Percy.
Even before Percival's premature exit, the characters were preoccupied by thoughts of death and decay, and Percival's death becomes a lightning rod for the characters' thoughts about all things Grim Reaper.
Falling Action
Gettin' Old is Hard to Do
The characters are now beginning to feel their age, and they all struggle to some degree with the onset of The Big D. (That's death, guys. C'mon.) They're pretty settled into their lives, with some (e.g., Louis and Neville) achieving stratosphere-level success. They note the difficulty of keeping up with old friends and initial awkwardness that happens each time they meet up.
Resolution (Denouement)
Surprise!
In the final chapter, as he attempts to sum everything up for an unnamed listener, Bernard suggests that these six different narrators might actually represent facets of a single individual. Gotcha!
At this time, Bernard attempts to resolve his love/hate relationship with language that has preoccupied him throughout the novel, ultimately deciding that continuing to wrestle with language (through communication and art) is the only way to struggle against the forces of capital D death. With this realization, Bernard closes the novel on a hopeful(ish) note.