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
Poirot finishes a case in Syria and boards a train to Stanboul. Suspicious activity commences.
Our setting (a train) is introduced, along with our most important character: famed Belgian detective Hercule Poirot. In this section, we get a sense of the strength of Poirot's reputation and the logic of his methods (observation, reason, etc.). We'll see his powers in action a little bit later. We also meet Mary Debenham and Colonel Arbuthnot, whose odd behavior on the train platform foreshadows the strange events to come.
Conflict
Poirot boards the Simplon Orient Express along with an incredibly diverse cast of characters. Mr. Ratchett is murdered.
The basic premise of any mystery novel can be summed up with one word: Whodunit? The second stage of the novel sets up the conditions that lead us to this question. The nasty Mr. Ratchett is stabbed twelve times in his bed, and as there are no police on board, Poirot is asked to take the case. The hunt for clues – and the murderer – begins.
Complication
Conflicting evidence is presented by the train passengers. It's all just a little too perfect, n'est-ce pas?
Poirot collects evidence and interviews the train passengers. The problem is that absolutely nothing adds up. Some of the clues appear to be planted, the time of the murder cannot be determined, and we hear tell of a mysterious, dark, womanish man. In the murder novel business, we would say that "the plot thickens."
Climax
Poirot uses his little grey cells and hits upon the solution to the crime. He must gather evidence to support his hypothesis.
In the climax of the novel, we get to see Poirot's logic and deduction skills in action. We know he's got it all figured out in his head, but we watch him actually returning to each passenger and confronting them with the truth: that is, that they're all connected to the Armstrong family. This section of the novel takes its sweet time, which, fortunately, allows those of us who are still trying to catch up with Poirot's thinking to try to figure out the solution for ourselves.
Suspense
Poirot exposes the passengers one by one.
As the plot unravels, the suspense builds. Suspicion is cast from one passenger to the next and eventually to all of them. Though we know who is involved, we still may not have put all of the pieces together. For that, we'll have to wait for the next section.
Denouement
Poirot offers two conclusions to the case.
For those of you who have no idea what's going on, the denouement of the novel offers sweet, sweet relief. Poirot offers readers two scenarios: one which is obviously false, and another which has the ring of truth to it. The analysis hits each aspect of the case point by point, allowing readers to stand back and admire Christie's intricately woven plot. Poirot delivers this section theatrically and with great flourish, as we get a sense of what a great detective he – and in turn Christie – is.
Conclusion
Justice is served.
M. Bouc and Dr. Constantine are asked to act as judges. They must pick the correct solution offered by Poirot. The novel's tension between justice and revenge is resolved here, at least partially, when the two men determine that the death of Ratchett was a case of serving justice, rather than a case of murder.