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.
Initial Situation
David's youth in Brooklyn and flight to Paris
It is David's first reminiscence that sets the stage for everything else that will happen in the novel. We learn that he grew up without a mother, and that his relationship with his father was strained. Perhaps the key event here is David's fling with a boy named Joey. Though he likes Joey, he refuses to accept the consequences of what that means. From this point on, David becomes distant from his family and perhaps also from himself. Both running from and searching for something, he decides to leave Brooklyn for Paris.
Conflict
David's relationship with Giovanni
Before Giovanni, David is a tease. He lets Jacques, an older gay man, buy him drinks, loan him money, and fawn over him, but David never goes any further. Though he hangs out with a bunch of gay men, he acts as though he's really interested in women. In fact, we learn that he has a girlfriend named Hella that he proposed to before she went to Spain. The attraction between Giovanni and David is so intense, though, that it consumes him. He cannot help but get entangled with Giovanni, and the result is that, as he waits for Hella to return, his situation is ripe with conflict.
Complication
Hella returns and accepts David's proposal
The complication is inevitable. Sooner or later, Hella is going to come back from Spain. David knows this and tells Giovanni, but in a way they both continue their relationship in a state of denial. Giovanni seems to harbor a secret belief that David could never actually leave him, and that the two of them might make a life together. Shortly before Hella returns, Guillaume fires Giovanni. Giovanni is desperate and entirely dependent on David, so that when Hella does return and accept David's proposal of marriage, the love triangle is set and it is fit to explode.
Climax
David leaves Giovanni
After a tense encounter in a bookstore, David knows that he will have to return to Giovanni's apartment and announce his plans to leave him. Yet he has no idea how difficult that will turn out to be. He finds Giovanni in tears. Giovanni screams at him, calls him a liar, and accuses him of never having loved anyone. Giovanni also reveals a terrible secret from his past that has driven him into his current situation of desperation and despair. David listens the entire time, feeling strangely numb, yet it seems clear that Giovanni is finished.
Suspense
Giovanni falls down the social ladder, and then kills Guillaume
As David tries to make a life with Hella, he also makes an effort to keep tabs on Giovanni. It is apparent that things are not going well for him. First, he lets himself be seduced and taken care of by Jacques, and then he begins hanging out with the poor street-boys that he used to despise. The dreadful question both David and the reader have is: what will happen to Giovanni? The question is answered a week later, when Guillaume is found strangled in his office.
Denouement
David's relationship with Hella falls apart as he torments himself over Giovanni
Up until this point in the story, the downward spiral has been Giovanni's. David has been standing on the periphery, doing everything that he can to keep in control of his own life. But when he learns that Giovanni will be executed, guilt overwhelms him. At the same time, he begins to admit that he does not love his fiancé, that he is not even attracted to her. Eventually he goes out carousing for men, and when Hella finds him it is clear that their life together is finished. David has been caught in Giovanni's downward spiral, which he might have helped to produce in the first place.
Conclusion
David, alone, leaves the house in the south of France to continue his life
The novel ends as it began, with David alone in the house in the south of France that he and Hella were supposed to share. He has now confessed everything that happened, and he is about to leave the south, to head back to Paris and, perhaps, one day, to America. The only concern now is whether or not David will ever be able to overcome his guilt and come to terms with his own sexuality. The ending is deeply ambiguous, but tormented as he is, David is not without a sense of hope.