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
King Louis's twin brother languishes in the Bastille.
They say information is power, and when the novel opens, this very tantalizing piece of information has made its way to the already-powerful-but-always-scheming Aramis. Swapping King Louis XIV for his twin brother would be a major political coup for Aramis. He could ascend to the papacy, save his friend Fouquet, and be one of the King's top advisers. The conditions are right for change, and that's exactly the requirement necessary for us to identify an Initial Situation.
Conflict
Twin brothers are pitted against one another.
For five minutes in Chapter Twenty-Four, King Louis XIV and his brother Philippe have a showdown in the King's bedroom. It goes along the lines of, "I'm the real king!" "No, I'm the real king!" The conflict here isn't just between the two royal twins, but also concerns the reactions of court members when they learn of Philippe's existence. Upon seeing Philippe, Fouquet faces pangs of remorse for siding with Louis. Anne of Austria is clearly distraught when she comes face to face with the son whose existence she has denied. Even D'Artagnan is taken aback during those shocking five minutes that the twins stand face to face. His internal conflict is weakest, however, as he almost automatically reacts and arrests Philippe.
Complication
Fouquet saves the real king but is in danger of arrest anyway.
Fouquet panics when it becomes clear that although he single-handedly saved King Louis from going mad in the Bastille, he's going to get arrested anyway. The situation is made even more complicated because D'Artagnan doesn't want to arrest Fouquet and keeps trying to circumvent the King's orders.
Climax
Athos and Porthos kill over a hundred men as they make their escape from Belle-Isle.
There are many climactic points in the novel (a product of it being published in serial form), but if this were a movie, this would be the big battle scene. Two men versus an army. Talk about being outnumbered. What gives the sequence added poignancy is Porthos's death at the very end as he struggles against the weight of several tons of rock with Aramis, seconds away from making his escape. He is the first of the four friends to die, and in a sense this is what we've been building towards throughout the book – the end of an era.
Suspense
D'Artagnan and the King engage in a battle of wills.
We're nervous for D'Artagnan as he storms back to Nantes in a deadly rage. It's clear that nothing ensures safety with the King (you've only got to look at poor Fouquet).
Denouement
Porthos has a funeral, Athos dies, Raoul dies.
If you give us a moment, you'll see that this resolves the suspense created in the earlier stage. The King wins the argument with D'Artagnan by pointing out that he is king. In other words, he is an absolute monarch and D'Artagnan has no business trying to manipulate or sidestep him. As we argue in "What's Up With the Ending?" this type of absolute sovereign has no use for either the strength of Porthos or the dignity of Athos. By killing off the two friends, Dumas simultaneously glorifies the pre-Louis days and critiques the post-Louis days.
Conclusion
D'Artagnan becomes a marshal of France, then dies.
Death is always conclusive, isn't it? See "What's Up With the Ending?" for more thoughts.