For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
After years of political infighting and war, the Costaguana-born Englishman Charles Gould is hoping to bring lasting peace and stability by starting up a silver mine. The plan goes okay for a little while at least—he gets rich and becomes powerful, as do his friends/business partners…
Act II
Eventually, certain people within Costaguana's political stratosphere get cranky about the fact that foreigners are gaining all the political power. General Montero, the Minister of War in an administration that Gould helped bring into power, leads a revolt against the current President. Nostromo, who is known around town as a go-to guy, gets heavily involved in defending the Goulds' interests and fighting off the Monterist invasion of Sulaco.
In the process, he helps get the Goulds' silver supply out of town so invading forces can't seize and use it to help their cause. A local journalist, Martin Decoud, is also involved in that caper (he also comes up with the idea to have the Occidental province of Costaguana secede to achieve lasting peace).
Act III
Thanks to Nostromo's efforts, General Barrios and other anti-Monterist forces manage to fight off the revolutionaries. Martin's dream of secession comes true, and the Occidental State comes into being. A happy ending, right? Nope, not so fast. This is a Joseph Conrad novel we're reading, guys.
Unfortunately, Martin died before he could see the Occidental State thing happen. Meanwhile, Nostromo holds on to the silver he had rescued, because the Goulds think it's at the bottom of the harbor. Unfortunately, his obsession with protecting the treasure makes him a little batty, leading him into some dicey decisions. He ends up getting accidentally shot as a result. Greed sucks, everyone. Just say no.